1. Staples coupons expiring Saturday, 7/31/2010. YMMV on usage and stacking.

$25 off $75 breakroom and cleaning coupon, online and phone only, code 29250
15% off coupon on entire order, stackable

Write down $25 off $75 coupon code on a piece of paper and tell cashier you have a code you'd like to use after he/she rings you up.

2. Items to buy @ Staples B&M

-Iris 60-quart buckle-up box, transparent blue #781454 $4.99
-Staples Hype! pen-style assorted highlighters, 6/pack #412783 $0.25 (limit 2 per purchase)

3. Buy 15 Iris boxes and 1 Hype! highlighter to reach $75

15 * $4.99 + $0.25 = $75.10
$75.10 - $25 ($25 off $75 coupon) = $50.10

Most likely scenario:
$50.10 - $7.52 (15%*%50.10 discount) = $42.58 + tax

$42.58 - $0.25 = $42.33
$42.33/15 = $2.82/box

Unlikely scenario:
It's possible, like the $25 off coupon when stacked with the 20% off 1 item coupon in the Staples Free Backpack deal, that 15% is taken off pre-$25 off $75 discount. In that case,

$50.10 - $11.27 (15%*$75.10 discount) = $38.83 + tax

$38.83 - $0.25 = $38.58
$38.58/15 = $2.57/box

Will do this on the last day, Saturday.