You can create an invoice programmatically in GetPaid. The general syntax to create an invoice is: 

wpinv_insert_invoice( array $invoice_data = array(), bool $wp_error = false );

Parameters

• status (string) (Optional) The invoice status. Learn more about invoice statuses.
  • Default: ‘wpi-pending’ 
• user_id (int) (Optional) The invoice user id.
  • Default: user ID of current logged user
• post_date (string) (Optional) The date-time in Y-m-d H:i:s format.
  • Default: Default is the current time.
• cart_details (array) (Required) The invoice items details.
• id (int) (Required) The item id.
• quantity (int) (Optional) The item quantity.
  • Default: 1
• custom_price (float) (Optional) The item custom price. This price will overwrite the item price.
  • Default: empty
• meta (array) (Optional) The item meta. This stores custom values that to be used for this line item.
• payment_details (array) (Optional) The invoice payment details. Required for paid invoice.
• gateway (string) (Optional) The payment gateway name. Required for paid invoice.
  • Default: ‘manual’
• currency (string) (Optional) The currency code (ISO 4217). ex: USD
  • Default: Store’s default currency.
• transaction_id (string) (Optional) The payment transaction id for paid invoices.
  • Default: Current invoice ID.
• user_info (array) (Optional) User billing address details.
• first_name (string) (Optional) Billing address first name.
  • Default: User’s saved first name.
• last_name (string) (Optional) Billing address last name or surname.
  • Default: User’s saved last name.
• phone (string) (Optional) Billing address phone number.
  • Default: User’s saved phone number.
• address (string) (Optional) Billing address street name/street no./block no etc.
  • Default: User’s saved phone address.
• city (string) (Optional) Billing address city.
  • Default: User’s saved city.
• country (string) (Optional) The country code (ISO2). ex: US
  • Default: User’s saved country or store’s default country.
• state (string) (Optional) The state name or state code.
  • Default: User’s saved state or store’s default state.
• zip (string) (Optional) Billing address postcode.
  • Default: User’s saved postcode.
• company (string) (Optional) User’s company name.
  • Default: empty
• vat_number (string) (Optional) User’s vat number.
  • Default: empty
• discount (string) (Optional) The coupon code to apply discount.
  • Default: empty
• due_date (string) (Optional) The invoice due date in Y-m-d. Only for invoice which has payment pending.
• ip (string) (Optional) IP address of the system from which invoice created/paid.
  • Default: IP address of the current system.
• user_note (string) (Optional) Short message or note for user. User notes will be included in user invoice email sent to users.
  • Default: empty
• private_note (string) (Optional) Short message or note for system use. Only admin can see private notes from backend invoice page.
  • Default: empty
• parent (int) (Optional) For renewal payment only. Parent invoice ID for which the renewal payment is being paid.
  • Default: 0
• $wp_error (bool) (Optional) Whether to return a WP_Error on failure.
  • Default: false
• Return # (int|object|WP_Error) The value 0 or WP_Error on failure. The WPInv_Invoice object on success.

Examples

Create an invoice with simple data with status “pending payment”. (GitHub)

$data = array(
	'status'        => 'wpi-pending', 
	'user_id'       => 1,
	'cart_details'  => array(
	array(
		'id'    => 2118
	),
) ); 
$invoice = wpinv_insert_invoice( $data, true );

Create an invoice with advanced data with status “pending payment”. (GitHub)

$data = array(
	'status'            => 'wpi-pending',
	'user_id'           => 1,
	'cart_details'      => array(
	array(
		'id'            => 2118,
		'quantity'      => 1,
		'custom_price'  => '100.00',
		'meta'          => array(
		'post_id'   => 1239
		)
	),),
	'user_info'         => array(
		'first_name'        => 'GD',
		'last_name'         => 'User',
		'phone'             => '+91 1119348834',
		'address'           => 'Kankariya Lake',
		'city'              => 'Ahmedabad',
		'country'           => 'IN',
		'state'             => 'GJ',
		'zip'               => '380002',
		'company'           => '',
		'vat_number'        => '',
		'discount'          => '10P'
		),
	'due_date'          => '2017-08-10',
	'ip'                => '1.39.51.23',
	'user_note'         => 'This is user note.',
	'private_note'      => 'This is a system note.',
);
$invoice = wpinv_insert_invoice( $data, true );

Create an invoice with simple data with status “paid payment”. (GitHub)

$data = array(
	'status'            => 'publish',
	'user_id'           => 1,
	'post_date'         => '2017-08-08 11:20:38',
	'cart_details'      => array(
		array(
			'id'        => 2118 ),
		),
	'payment_details'   => array(
			'gateway'           => 'paypal',
			'currency'          => 'USD',
			'transaction_id'    => '9TK485797B9101342'
		),
	);
$invoice = wpinv_insert_invoice( $data, true );

Create an invoice with advance data withthe status “paid payment”. (GitHub)

$data = array(
	'status'            => 'publish',
	'user_id'           => 1,
	'post_date'         => '2017-08-08 10:10:15',
	'cart_details'      => array(
		array(
			'id'            => 2118,
			'quantity'      => 1,
			'custom_price'  => '100.00',
			'meta'          => array(
				'post_id'   => 1239 ) ),),
	'payment_details'   => array(
	'gateway'           => 'bank_transfer',
	'currency'          => 'USD',
	'transaction_id'    => 'BNK443545454557' ),
	'user_info'         => array(
		'first_name'        => 'GD',
		'last_name'         => 'User',
		'phone'             => '+91 1119348834',
		'address'           => 'Kankariya Lake',
		'city'              => 'Ahmedabad',
		'country'           => 'IN',
		'state'             => 'GJ',
		'zip'               => '380002',
		'company'           => '',
		'vat_number'        => '',
		'discount'          => '10P' ),
	'ip'                => '1.39.51.23',
	'user_note'         => 'This is user note.',
	'private_note'      => 'This is a system note.',
	'parent'            => 0 ); 
$invoice = wpinv_insert_invoice( $data, true )