what if this happen

curse them?

Was in that situation a year ago, after i contact the winner he marked it as 'Not recieved'.

Contacted SG support with screenshots + Steam Gift send log.

They tell me 'We just solved this problem by delete your giveaway', so yeah. Solved.

Contact Support, ideally with some proof.

They'll sort it out for you :)

If after 7 days they don't mark it received you report them by hitting the support button. Make sure you have screen shots uploaded to a image site already to give to them as evidence that you did send it.

My suggestion would be after a day or so if they have not hit received yet, I would try to contact them via steam. They might not know they got an e-mail or anything from you yet. Ask them if they received it and if they have remind them to hit received on steamgifts.com

In the future if you are giving a serial key I would add them on steam friends and give it to them that way when they are actually there talking with you, and that way you can remind them to hit received. This is what I usually say.

"Greetings and congrats for winning my steamgifts giveaway."
Once they respond I continue.
"Give me a sec and I will get you your key."
This is when I go and get the key and open up steamgifts.com to the created page so that I can get ready to do stuff on my end.
I then send them the key via chat, I also hit sent on steamgifts.com
I then say.
"Here ya go, don't forget to hit received on steamgifts.com once you have a chance."
99% of the time, they go and do it right then and there. The other times steamgifts.com was acting up.
Once they say they have, I check the created page to confirm then I respond with.
"Again congrats on winning the giveaway, enjoy the game. :) "

I actually do this for stuff in my inventory and for serial numbers. That way you know for a fact that got it.