Matt Thompson has some advice for you: stop buying cheap-ish pseudo-generic drugs from Walgreens, Rite-Aid, and Duane Reade and start buying really cheap true generics.
As you might know, Benadryl (available at Walgreens.com for $5.29 for a box of 24 capsules) and Wal-dryl ($3.99 / 24 capsules) are otherwise known as "25 mg. of diphenhydramine HCI." Compare [with the true generic available at Amazon]. Yes, that is 400 tablets containing 25 mg. of diphenhydramine HCI, for about $10 when you factor in shipping.
Heed his words. Here's 300 tablets of generic Claritin for $11.00, 100 tablets of generic Zyrtec for $6.99, 240 tablets of generic Zantac, 1000 capsules of generic Benadryl for $20.34, 1000 tablets of generic Advil for $11.70, and 1000 caplets of generic Tylenol for $13.91.
Update: It's been brought to my attention that the Kirkland brand is Costco's store brand so any Kirkland products sold on Amazon are being resold by people buying them from Costco. (thx, ivan)