paying for subscriptions outside Membership

I'm building a membership framework for a client who does not want to revoke any memberships just because current members haven't paid their renewal fees on time. In other words she wants to be really generous with allowing people to renew late. The way I thought I could do this is by decoupling the renewal payments from the membership renewal process. The site has a regular Woo-commerce shop and I thought I could just send people to the shop to pay for their memberships and then the client can use shop notifications to determine whether to terminate a member or not. So I just set everyone on serial renewal subscriptions with zero renewal fee and they just keep renewing automatically until the client drops their subscription because they haven't paid through the shop. But for this to work I need to be able to redirect new registrations to the shop to pay their fees. I want the 'Register My Account' button on new sign-ups to re-direct to a shop page after adding the new member to the database. How can this be done?