For new customers only, Checks Unlimited cuts up to 75% off select checks via coupon code "TGY7". With free shipping, that's tied with our mention from two weeks as the best discount we've seen from Checks Unlimited. (It typically only takes up to 70% off.)
A best bet is one box of Scooby-Doo Mystery Inc. Checks (pictured) for $21.99 before the above coupon, $5.50 after. (Search under "Special Edition Checks" to find this style.) With free shipping, that's $16 off and the lowest total price we could find.
Note: Up to four boxes of checks can be purchased at this per-box price.
Before you send in your tax return, double-check that you haven't made one of these mistakes that can trigger IRS scrutiny.