usually you apply the discount at the shopping cart stage, the price is then recalculated at checkout, could be done that way but it's a little cumbersome.
One way to implement a token-based discount is that whoever owns/holds a token gets the discount. Another way (that I had in mind) requires you to transfer it back to the store. In this case it is lot more convenient to transfer both BTC and Counterparty token in the same transaction.
theoritally it's fine to replace the antidust amount sent to the recipipient with any amount larger. However, it's not implemented in the current counterpartyd yet. Maybe later we can customize the amount of BTC we sent in xcp transactions, but I am not sure whether this is really useful.
It actually is implemented, both in the CLI and the API, at a per-transaction level:
Cool, I believe such transactions will be common in the future.
An example is Alice the Musician. She sells her mp3's for 0.05 BTC each. She realizes that she can reward fans who blog about her, write in forums, etc, by giving them a discount token, ALICECOIN. One Alicecoin is backed by a 0.01 BTC discount.
When buying with discount, the buyer transfers 0.04 BTC + 1 ALICECOIN in one transaction.
A huge advantage for Alice is that there are no direct costs with issuing the coin, but the fans who anyway would buy mp3's see a real value in it. Holders of the coin who do not want to buy her music - even with a discount - will try to sell it on the DEX. If she gives away a reasonable amount of coins the market price should be slightly less than 0.01 BTC.