I'm using GT SIM now since January this year in a lot of countrys in Europe, i get two numbers from them and do not get any problem to get my SIM cards.
It can happen that the post company lose something.
I know that GT SIM support is not perfect and also not really easy to reach.
Then send them a LOT of mails with the reference of the past mails. Sometimes it works. In the same time call them (if you use their service directly from their website it doesn't cost you anything).
In my case sometimes they charge me bad and i have to do it like that, but at the end they always give me my money back.
Now i suggest you not to leave this like that YOU SHOULD GET YOUR SIM with no extra cost,
then ask them again and again and again 
to send you the SIM that you already pay.
I was also a UNITED MOBILE customer and i think that GT SIM is a good deal. They offer you a voice mail by web (no cost), you can buy DID numbers then the people can reach you on a local non mobile phone number and you only get charged 0,10 Euro/min(if the country is free in incoming calls).
You can send web SMS and web calls (BETAMAX is cheaper
) evrything with the same bill
Hope this can help you
luciano