[Nyx]

Thanks for the advice. Will not go the damaged item route then.

So just to clarify the process:
1.  I order the 2080Ti and wait for it to be delivered.
2. I contact Amazon to say that I've received a 1660 instead, which is a wrong item.
3. Send the 1660 in its full box to Amazon
4. They will accept it and refund me for the 2080Ti

I've also checked on Amazon and I've realised that the weights of these two GPUs are not exactly the same.
1660: Item Weight - 1.96 pounds, Shipping Weight - 3 pounds
2080Ti: Item Weight - 1.46 pounds, Shipping Weight - 3.89 pounds

Their boxes are identical though except for the model number on the box (GTX 1660 vs RTX 2080Ti). Will this be an issue?

Will this method work for such a high-value item and should I be using the same account/address to order both GPUs?

Weights are a fair bit off each other, see if you can stuff the packaging with something they wouldn't notice to match weight (eg. cut a slit in the foam material the GPU sits in and fill it with something). Also no, you wouldn't want to use the same account. Order it on 2 completely separate accounts to 2 different addresses. If you can't do 2 different addresses, order the false GPU off another site.