You don't have an on-call rotation but do have people dedicated to vendor relationships, and that guy works on the weekend? I'm not sure how you completely avoid getting alerts on the weekend for third-party payment processor issues, which can happen anytime, if you actually want to transact business on the weekend.
I don't know what exactly GP works on, but 'bank' covers a lot of ground and some things happen on weekends. 'Open Banking' APIs, and some payment rails for example.