Goodwill's income goes to helping people get jobs and opportunities they might not otherwise have. Would you feel guilty using a coupon to save money on your purchase there?

No. The coupon was designed to get people into the store. The things they sell have been donated (no cost), their markup, though reasonable, is enough to pay for all of their operations; And a goodly amount of their merchandise has been donated by ME!

I've only been to a goodwill a couple of times but if they offer a coupon why would I not want to use it?

I do not believe Goodwill accepts any form of outside coupon. Goodwill sometimes offers their own in-store coupons for certain dates. There is no tax on used clothing and they give seniors, age 60 and above, a 10% discount.