OfficeMax recently shared some teacher-focused coupons for their ImPress Print Center that you can print out from this Google Doc. The first deal has expired (I apologize for not sharing these sooner, but school has already started), but there are two coupons that run through September 24th:
$8.99 on 18”-by-24” posters (regular price is $17.98)
50% off on lamination services
Download the coupons here. I hope you are looking to avoid too many out-of-pocket expenses, but these should help if you can't avoid it!