How do you charge for custom features / integrations for your SaaS?

Hi all, I have a SaaS (in this case a Shopify app) and a prospective customer reached out asking if I could do a custom integration for them. My thought was I could charge a one-time development fee (without guaranteeing success), but then use those funds as a credit toward future app subscription fees - essentially doing it for free if they pre-pay X months. Does that feel right or am I selling myself short?