How will you guys know to send it to my imvu account if you guys don’t have it?

Question: How will I receive my products?

Answer: Every product comes with all of its raw assets and access to our private product server, with software and step by step instructions on how to access products on our private product server. We use a private product server that we control instead of gifting directly via IMVU. This protects our users and their products, removing IMVU staff and IMVU's review system from the equation. Products come with software for accessing products on our private product server during normal chats with your friends / sex partners on IMVU. Gifting / selling from our private IMVU product server is permitted and will not cost you any credits. You may use our products in any room / chat, with anyone you want. AP is not required. A secondary IMVU account is recommended, but not required.

Giving us your ingame username would be a security risk. If our server ever got confiscated ... the last thing you want on it is a bunch of IMVU usernames sitting in our database. Instead, you will press a "use product" button on our site ... and it will simply send the product to whatever IMVU account your IMVU client is currently logged into, or you will give a gift code to your partner if the product is for them.

Source: http://www.imvumafias.org/community/faq.php#f0r1