Oh hey, I saw the cat signal.
Uh, yeah, you can't avoid conflict here, so it's really just a matter of picking the boundary you want to defend.
The thing is, the extra $28.doesn't matter and isn't the point, so forget about that for a second. The conflict you need to address is the person spending more than you agreed to in the first place and that you don't want this to passively become the standard.
First, always give friends the benefit of the doubt that they're reasonable people. If they aren't...don't be friends with them. So if these two other people are reasonable, then talk to them, like any normal, respectful adult would, and say that you are not cool with being asked to pay double what you agreed to, and don't want to set a precedent that everyone needs a $150 gift for their birthdays.
These are not unreasonable statements to make to friends who supposedly like and respect you.
Might you piss them off? Maybe.
Might they decide that you're cheap? Possibly
Might they gossip about you when you're not around? Newsflash, they already probably do.
Should any of that impact you? Absolutely not
If it were me, and this was the first time this happened, I would personally just pay the extra $28, but make my feelings and expectations very clear, not getting mad at anyone, not blaming anyone, just graciously contributing and setting firm boundaries as to what you will and will not do in the future.
I'm not saying you should do that, I'm sharing an example of how what you do with the extra $28 is utterly irrelevant to how to address the issues of boundaries.
You can't control how people will react, you can only be as true to your values as possible, which is to show that you care, but that you also won't be made to spend money didn't agree to ever again.
This really isn't as complicated as you think it is. You only think it is because you're trying to figure out how to avoid conflict, when instead, you should be embracing the opportunity to engage in really constructive conflict with people who have no interest in hurting you.
The key to really constructive conflict is to not focus at all on worrying about how the other person/people will react, but instead on how you want to make them feel when you talk to them.
So if your goal is that everyone feel respected, then focus on showing that what you need is respect, and what you are giving in return is also respect.
Really, don't worry about pissing people off. Pissing people off is not the end of the world. If you are coming from a loving place, they'll get it and respect it if they even give half a shit about you.