Apparently they make more money from memberships than from sales margins, which are capped.
I left reddit after the API fiasco and didn’t return, but I remembered seeing this on Data is Beautiful before that, so I just looked it up for you. (My search query in DDG was “data is beautiful reddit Costco profit margins” and a few popped up; this was the most recent.)
Transcription: infographic states
Net sales +$77.4B
Merch costs -$69.2B
Membership fees +$1.5B
SG&A -$6.9B (Selling, General, and Administrative Expenses)
Taxes -$0.8B
Net profit $2.2B (2% margin)
Source listed as: Costco Q4 FY 23 earnings
Edit: format failing :(
Last I heard, their profits were nearly entirely from memberships. This was probably five years ago though. I don’t know if their numbers have changed since.