Please don't throw them away. Here's a link with a few addresses of overseas military installations that can distribute them to service member families. Commissaries will accept manufacturer's coupons for up to 6 months after the printed expiration date.
I have heard that it's helpful to separate food and non-food coupons into ziploc bags marked as such. I think the best way to send them would be to gather from a few people (maybe have everyone from church put them together) and send them in a fixed rate box. The addresses are APO boxes, so you have to send them via USPS.