I am not a lawyer, but I found relevant information. Reproduction of copyrighted material is completely fine if it is considered Fair Use under the Fair Use Doctrine. Th doctrine provides a litmus test that you can apply to determine if your use would be considered fair. Below are the relevant clauses.
... personal use is favored for fair use, while commercial use is disfavored.
Unless you plan to distribute these cards or sell them to your friends, you pass this test.
... Use of imaginative works is more likely to require permission ...
You fail this test. The entire card is a creative work.
... Using only a small portion of a copyrighted material tips towards fair use, while using large portions indicates a need for permission
This doesn't really apply. The rule was intended for books, movies, music, etc.
Where a work is available for purchase or license at a reasonable cost, copying all or a significant portion of the work (in lieu of purchasing or licensing a sufficient number of "authorized" copies) would likely be unfair.
You fail this rule, because the cards are available for purchase.
The rules are obviously vague, and different states apply them differently. Only Wizards can give you peace of mind. All of that being said, you are safe printing proxies because they are banned at tournaments, they are worthless anyway, everyone does it, and even though Wizards strictly prohibits it, they have never gone after anyone for it.
As for what you can tell CostCo, you don't have a case. If you tried to make one, you would fail rules 2 and 4. Go print at another location, or just write over your basic lands like everyone else.