I would store my data in a database that I control, so that it cannot be disconnected or altered by a service agreement change, or an API "upgrade".

I would ensure that any payment processing can be done through a range of processors, so that if any one of them becomes unavailable my store is not affected and I can continue taking payments.

I would also route payments through the processors dependent on cost. Some rails are more expensive for particular transaction types or countries, etc. Being able to switch between them at will is extremely useful.