Yes to both of your questions.
Payment pending means that the shop has been approved by the MSP and they are in the process of getting you paid. However, if you have not read in your contracts, most MSPs take 30-45 days to make a payment, so it might be a little while before you are paid. But you will get paid.
If a shop gets declined, you will know that too. And you will not be paid. If a shop is declined, there will be no payment pending.
Whether approved or declined, you will receive an email indicating which way the decision went. Once a decision is made, no further review is necessary.
iSS does not approve or decline shops, or make payments to you. The MSP is responsible for all of these decisions.
iSS is a shopper portal or platform that contracts out to MSPs to place job offers for their clients on this portal and make them available to us, the shopper. Think of it like a grocery store; food is sent to the store where it is put on shelves for us to buy and take home. iSS works the same: the MSP sends shops to iSS, iSS then lists the available shops on the platform (website), and we the shoppers come along and pick and choose shops to complete and get paid! Nice, heh!
Hope that helps and happy shopping!