Where do all these coupons come from??

Yep

kk thanks a lot

They were "prizes" doing achievements (you had a chance of a game, a piece of coal or a coupon) and crafting 7 coal (which also resulted in either a game or coupon).

Towards the end of the event Steam "ran out" of 3rd Party games and coupons, so they gave out valve coupons, valve games and TF2 items.

Oh, sadly the event is over, and doing the achievements wont gain you any of the rewards. :(
And no I don't, the coupon is all yours if you want it.

edit sent you an invite on steam.

The all thing with the coupons is over now,you can't do those achievements anymore because the event(The Great Gift Pile) is over.But you can find free coupons here

Just a friendly warning, maikimaik. While Thrilhouse was kind enough to give you their coupon, don't ask again. The mods consider it begging and will ban you.