Question on grading and shipping

Hi ya'll never graded a card before but i wanted to ask for some help in regards to getting some pokemon cards graded. I have a few cards i wanted to submit to psa later on and would like to know how to get started with it. I have a mega charizard sir from pf, mega darkrai sir from pb and a few other cards in the $50-$90 range raw i believe. For shipping am I able to hip them all together to be graded and how would i go about doing that. I know psa has grading/shipping options based on potential value so would i choose the option for total value for all my cards or just 1 at a time? I noticed the lowest option currently is $80 so i was wondering if during checkout or after i select all the cards i have would I need to submit each one by itself or will it give me the option to choose what value each card will be. I also heard psa also upcharges depending on the grade you get especially if you think the card will be a 9 but they value it at a 10, would there be a scenario where if i pay too much for a card to get graded they refund me partially if it gets a lower grade than what i expect it to be/paid for.

Thank you for the help ya'll