Anything that is businesslike and patriotic. I refuse to accept anything seasonal or cutesy, and lets not EVEN get in to the RoP or various "minority" commemoratives.
I'll never forget the time one January the postal clerk was trying to talk me into taking the leftover Madonna and Child Christmas stampsin lieu of the sold-out standard "flags". I finally drew up to my full diminuative height and announced that "I do NOT observe Christmas, but I DO honor our nation's flag!"(no I'm not Jewish, just Deitistical
)
The clerk's jaw dropped because I reckon she figured
all women were ga-ga over Christmas, or at least saccharin 'fine art' paintings of 'mother and child'. She was lost for words, said no more, but I DID(!) get my flag stamps, even though she had told me right off the bat that they were "out" of them
I have a negative quantity of respect for the USPS, and use Fedex or UPS every chance that I can reasonably do so.