Our system has no way of handling recurring payments such as a periodic subscription or installment plan; however, you can use PayPal Subscriptions with PayPal's own Subscription buttons at the same time as using E-junkie with PayPal, and they won't interfere with each other at all.
You may wish to combine that with a purpose-built membership management system such as FrontDeskApp, CheddarGetter, MemberWing, Digital Access Pass, etc. If you set up IPN manually in your PayPal account settings for any recurring payments solution, E-junkie's buttons would still override those IPN settings for orders placed through our system, yet without affecting IPN for any subscription or other payments unrelated to E-junkie.
The main problem with supporting such a feature at our end is that, even if we could send orders to checkout that would initiate a recurring payment plan, we have absolutely nothing built into our ecommerce system that could provide buyers and sellers with anything of value for those periodic payments, such as tracking membership/subscriber status, administering access to a members-only system, scheduling mailings/downloads, etc.
While we do use PayPal Recurring Payments to manage subscriptions for the use of our own services, that's entirely separate from our cart and order-processing system. We just use PayPal's standard subscription links to help our sellers set up a payment schedule in their own PayPal account, telling PayPal to send us the same amount every month, so we just passively receive those payments to mark the seller's account as active for another month.