Customizable Ecommerce Integration Solution

I am on the lookout for expertise to advise, design and integrate an ecommerce site. I'm unsure at the moment if using existing platforms would suffice or if a custom solution is necessary, but we do not want to get locked in to a subscription based platform if we can avoid it, so someone experienced with those (e.g. Shopify, WooCommerce, Magento), and custom solutions will be invaluable. The functions I definitely need are: - Integration with a fulfillment service provider (is there a standard ?) - Payment gateway integration - Inventory management Skills and experience in these areas are an absolute requirement. If you can assist in making this decision and implementing the solution, please make a bid.