The source I gave will send you 5 lbs for $18.90 postpaid and no other purchases required, so if that's the quantity I wanted, it would depend for me on whether I was going to buy something else at Amazon anyway, or not, so I could save the $0.90. The 10 lb price from Duda gets you the second 5 lbs for just $8.09, postpaid, and I have trouble resisting that kind of bargain. Duda had my order to me in under a week, if that matters to someone reading this.
I also ran into a food wholesaler recently who had even lower prices, but I don't believe the shipping was free. Same with
this soap making supply site. Price is great, but it looks like shipping will take it past Duda's price.
One interesting point on the soap supply site is the claim citric acid will lose potency in a few months if not stored in airtight containers. That's not been my observation. All the other data I've seen suggests cool and dry storage only to prevent caking, because the citric acid is slightly hygroscopic and will pick up about ½% water in high humidity. Commercial brass cleaning solutions that use citric acid have indefinite shelf life, AFAIK. So as far as deterioration goes, I believe it is very stable. It made me wonder if the soap site warning was taken from ascorbic acid, which oxidizes gradually when moist.
Anyway, if you have doubts, get a pick pickle jar with seal and keep it in there.