Accepting payments on behalf of customers?

Let's say that I'm selling a service for a niche industry to accept reservations. What would be the best way of handling payments if I want end users to pay through my app?

I've seen options for TPPA (Third Party Payment Aggregators), which seems like what I'm looking for. So far, most sources say that this isn't the best approach because I would assume the responsibility for the transaction.

I believe the alternative would be to ask my business customers to set up a merchant account for my app specifically, right? That or I would need to be able to support multiple types. This also seems less than ideal, because it's another hurdle for making a sale.

You can tell this isn't an area of expertise for me - I'd love to hear your thoughts.