Need to use 3rd party processing and multiple warehouses. Possible?

Hi, 

I have a distributor that doesn't have a B2C eCommerce so I have to do it 

I already host my website (without any commerce at this time) on Squarespace so would like to possibly continue using. 

What they have is the payment processing and fulfilment etc. 

So what I'd like to know, is what do I need to be able to use their payment system in my eCommerce? I already have a Commerce basic account. 

I won't be charged anything if I use a 3rd party payment system, correct? 

Bonus question: I'd like to start selling accessories and items from other suppliers etc. Can Squarespace allow multiple suppliers/warehouses? Does it all go into the same basket or does it need individual stores? In other words, can I have a single store with items from different providers? Like for example, sell items from Printful, my distributor, and another local artisan's warehouse... 

Thanks!