You need to go more into details as your question is extreamly general. Whatever is your plan, what you can do to gain basic and higher level of understanding on how to do social media marketing is to follow tons of free tutorials/videos available in google/youtube.

Only after you get this kind of understanding you can move forward and look for experts to follow on Linkedin.

There is loads and loads of free, valueable info on how to get to know Facebook, Instagram or any other platform:)

Good luck!

Such solutions are wiedly available to current ecommerce business. Question you would have to ask yourself if what makes your different, what is unique in your product that is not currently available.

You are targeting (basically) 2 types of customers:
1. Those who already have a solution in place
2. THose who are opening their ecommerce business now (maybe due to COVID-19) situation.

You need to have selling strategy for both. By that I mean they way you find them, make contact and explain to them painpoints that you are looking to solve if they were to implement your solution.

Please also remember that when dealing with payment service, you are probably facing ton of regulations that you need to follow - I would be sure that you have that in place before you push further. Security angle is also very important.

Good luck!