When I order from them fairly regularly, I don't get this kind of coupon. It only shows up occasionally when I go more than a year without ordering something.
It's an e mail they send me where I click on the link in the e mail which takes me to the site, where it's already deducted from my cart at checkout. No code to punch in.
I'm assuming since the e mail says that they haven't heard from me in a while and here's $20 off on a purchase, that they send them out after you don't make a purchase from them in a while.
Other than replacing a worn out a piece of gear or purchasing water purification tabs, I already have everything I need that they carry, so don't order from them very often these days.
If you got one, it should be in your e mail. Just don't auto delete them like I do usually or you'll miss it. Sorry, I guess this only applies to customers they're trying to get to come back.