Just got one more drink ticket! Seems $280 works better as a multiple of $140 than $165 does.
FWIW --
Anyone who has not booked a meet for more than one person, but is used to booking for seminars, is likely to get $165 from the instructions if he's bringing a pillion and wants to get her a shirt, too. In seminars, it's the professional taking the seminar that is the "registrant" and it's common to book hotel space and to come to the no host cocktail parties, etc. with that significant other, for whom no registration fee is required. Make it clear that total number of persons attending in one's party (not motorcycles or forum members*) is what is referred to as "registrants" and I suspect you'll see the math problem disappear. Read it from that perspective (and not that of someone who has either organized dozens of M/C meets or who has attended with a pillion before) and you'll see what I mean.
* After all, the pillion probably doesn't have the required forum handle, and is probably the one for whom the different sized shirt is intended.