I actually am a Costco member. I've been getting this medication for over a year now and it's always been the same $19 amount. I've never been given the option of any other price.
You have to enroll in the Costco Member Prescription Program. It's free to enroll. You use this in lieu of your insurance for medicines that are cheaper under this plan vs. insurance, like most OTC drugs that your doctor can write a script for. You can't enroll in this program if you have a federally funded insurance like Medicare or medicaid. Just ask your doctor to write a script for generic Xyzal, then take it to Costco and use this program instead of your insurance. The price for a 90 day supply is $12.64.
https://www.costco.com/member-prescription-program.html