I use Stripe for invoices, but I can easily send an invoice through another platform if needed.
For processing transactions on the web, I would always lean toward using a service like ChargeBee that allows me to setup multiple payment gateways.
Getting off the ground quickly is one thing, but the moment that you have reliable revenue is the moment that you need to put some serious emphasis on redundancy across your business to plan for disasters, outages, etc. It's worth it to pay the fees to maintain a 2nd (or 3rd) payment processor once you have that type of revenue coming in.