Hi,
Unfortunately this seems to be a limitation of the CoinPayments platform, as the wallet is not meant for multi-user use. This only seems to affect monero.
The good news is that I have developed and am currently testing a full node Monero adapter that connects to the wallet RPC, so this will be a good way to provide Monero to users, albeit with a little more technical effort on the part of the site admin.
Once this adapter is released there will be a blog announcement.
kind regards