Finance
Everything you sell is in the same store.
A member joins the same way they take a conference place, pick up a document or order a jumper: one cart, one moment, one history to read afterwards. What they are charged is what their record already says they should be, and every credit and refund afterwards reaches finance without anyone re-keying it. Selling more to the members you already have stops being a project.
Sell a membership the way you sell anything else
One basket, one payment, however many things are in it. A renewal and a ticket go in together, and you see them as one purchase rather than two nobody connects.
Hand finance an order they recognize
The figure finance reports is the figure the member paid. A refund shows up in both places the moment it is issued, and nothing is keyed twice to make that true.
Integrations
Keep the processor you already use
The store takes the money, so it sits on top of whatever you already run. Connect the vendor you have and keep the rate you negotiated, or use the one that ships.
Payments
Dues, tickets, documents and merchandise are charged, retried and refunded through it.
An order that is placed, paid or refunded reaches your other systems through the REST API and webhooks that fire on what changed.