My sister picked something up for me from the store that I asked for.
When I asked her how much it was she said “it was around $2”. Can I pay her back $2 even if it might be more than it cost? Would it be a ribis issue?
(I don’t know if it makes a difference, but 1) she has sometimes given us maaser money/gifts 2) she would have been happy for me to take the item without paying her at all for it)
It is not a problem to pay the full two dollars. Even if the item really cost less than $2, you are giving her the $2 because you want to make sure that you paid her in full and not because of the loan, therefore it is permitted.
As a sidepoint, the fact that she usually sends you money, or that she wouldn’t mind giving it to you for free will not allow you to give her extra for the item that she paid for. This item was considered a loan, and you can’t pay her extra for, however as stated before you can give her the full $2.