createInvoice()
getInvoices()
Invoice subject is an object that has invoices (contact or estimate)
createInvoice() : \Moneybird\Invoice
\Moneybird\Invoice
getInvoices(\Moneybird\Invoice_Service $service, string $filter) : \Moneybird\Invoice_Array
\Moneybird\Invoice_Service
string
\Moneybird\Invoice_Array