Support additional payment methods for e-commerce orders (Invoice, PayPal, Google Pay, Apple Pay)

🔍 Problem Statement

As a client admin, e-commerce SKU orders can only be paid by immediate card charge (or BACS), so buyers who require invoice-based billing or prefer a wallet have no way to complete a purchase.

💡 User Story

As a client admin, I want orders to be payable via invoice, PayPal, Google Pay, and Apple Pay, so that buyers who need invoice-based billing or prefer another method can check out.

🎯 Definition of Done (DoD)

• ✔ Given e-commerce checkout, when a customer (or an admin creating the order) selects "Send Invoice", then an invoice is generated for the order amount and sent to the customer, payable by credit card or offline payment — and the order is fulfilled upon payment.
• ✔ Given e-commerce checkout, when a customer selects PayPal, Google Pay, or Apple Pay, then the order completes and is paid through that method.
• ✔ This change will impact UI / API — specifically the e-commerce checkout endpoint(s) and the default checkout UI's payment method selector.
• ✔ Limitations: each gateway (PayPal, Google Pay, Apple Pay) must be configured and enabled at the platform level.