For every US graduate program in mathematics I know of, the application process includes three letters. So evidently you won't be getting letters from most of the faculty you've worked with.
If I were advising a student who had done two successful research projects with two different faculty members, I would advise against getting letters from both of these faculty members unless each one can also discuss your coursework and progress through the major. In my experience, when it comes to letters for math grad school describing undergraduate research, the writers almost universally paint a happy picture of the work, which most often leads to an "all happy families are alike" situation. Namely, they (most often) say nice things about the student, their work and that potential, but not sufficiently incisive nice things to really help out the admissions committee. Which makes a lot of sense: if you gave a student a project to, say, determine whether each continuous map of surfaces such that the induced homomorphism of fundamental groups has a nontrivial kernel must then have a nontrivial element of the kernel represented by a simple loop and the student eventually got some partial negative results*...how do you compare that to other students -- no other student has worked on that problem.
Moreover, getting letters from more senior academics is (other things being equal) better than getting letters from more junior academics, because the more senior academics have more experience watching and guiding students through the various stages of their academic careers. I would think that a typical postdoc in mathematics is still more heavily informed by their own undergradaute and graduate experience than those of students they have been involved with.
Anyway, long story short: even if the postdoc had great things to say about you, you wouldn't necessarily want their letter unless they had the best things of anyone who could plausibly write for you. As this is not the case: don't get the letter from that postdoc, and don't worry about it at all.
*: This is in fact a description of my own undergraduate research. I did not get a letter from the faculty member about it, though I'm sure he would have had nice things to say.