I don't know if I understand the situation correctly. This is what I get from the OP's posts:
1. The original shop usually pays $90. The MSC offered a $30 bonus, making the total shop pay $120.
2. The MSC is refusing to pay the $30 bonus, saying it was a mistake.
3. The MSC won't budge on the $30 bonus even though it is their mistake. However, it is offering the OP bonuses on two other shops to make up for the $30.
This is how I would handle it:
A. Determine whether the bonuses now offered on two other shops would make up for the $30 bonus the MSC is not honoring. For example, what is the total bonus on the two new shops? Also, would I have accepted the two new shops without the bonus?
B. If I concluded the new bonuses would indeed make up for the $30 bonus, I would accept the new arrangement AFTER I got written confirmation from The Powers That Be at the MSC that CLEARLY set forth the new arrangement. I wouldn't want another "error" after I do the new shops.
C. Never forget what the MSC did. Get paid first. Then, decide how much, if at all, you wish to shop for them in the future.
I care very little how they choose to allocate/label the $30 bonus. I care about getting paid. Who knows? This MSC may have set limits on which shops/projects can offer bonuses and how much those bonuses can be.
As for whether the MSC may try to pull the same stunt again? I will cross that bridge when I get there, i.e. when a future shop offers a bonus.