Thanks, I am experimenting with Flutter at the moment. That is a very promising candidate for future all in one UI solutions.

Thank you @TeeAttack42 for the detailed answer. I am going to wait some time before I mark it. So, if I understood correctly, EOS.IO smart contract is capable of running the whole process, even to communicate with a merchant in case of other currencies to make sure the payment is done. The actual assets are off-chain and require real world trust (a signed contract), however digital signatures may be used to trigger the release of the money. Your answer is inspiring, thanks again.