Mid-sized e-commerce entrepreneurs advice needed

Hello guys, this post is not actually related to e-commerce itself, but i want your advice on something related;

I am gonna be having an interview next week as a Banking E-Commerce Relationship Manager, i will be handling a portfolio of existing merchants as well as acquiring new merchants, my main Product(bank side) will be the payment gateway solutions, I’m gonna be competing with like 20~30 financial entities that provide same services where I live, so, my questions here are;

1- What makes you guys choose between a payment gateway and another?

2- As a Relationship Manager, what do you guys think I should advise my clients with to encourage them to increase their sales volumes? (I am obliged to help achieve a target of total sales volume amount of $100 Millions per year on my bank’s payment gateway.

3-Most probably my portfolio will be of mid-sized e-commerce stores since sales volume target is too damn high(that’s 5billions in my local currency and out of total 70 billions industry size), any ideas how such stores work or strategize their marketing and sales efforts so I could provide them with consultations to boost performance accordingly?

Thank you all in advance, and again before any criticism, I am not sure if this is the right subreddit to be asking such questions, so excuse me.