If you read the fine print on the Visa card, it tells you that buying money orders isn't considered a purchase.
"What doesn’t qualify as a purchase?: The following types of transactions don’t qualify as purchases that earn Costco Cash Rewards:
o Use of checks that access your Card Account
o Balance transfers
o Cash advances
o Travelers checks, foreign currency purchases, money orders, wire transfers and similar cash-like transactions
o Lottery tickets, gaming chips and similar betting transactions
o Interest and account fees
o Unauthorized charges
o Items returned for credit"