Kajabi is an all-in-one platform that offers email marketing, online course hosting, memberships, and payments (to name just a few features).
Tax info:
How are my taxes filed with Kajabi?
Affiliate program info:
Buy buttons are available via Shopify integration:
How to Sell Physical Products on Kajabi with Shopify Buy Buttons
Landing page info:
Payment plan info:
How to Create a Multiple Payment Offer
Zapier info: