Damus does not hint the user if it fails to send a note and it is very confusing when it happens. I really like how noStrudel communicate this information clearly to the user.

I was about to say that I would like to see money back features when we pay with bitcoin/sats.

For exampel, if a customer pay 100 sats and 1 month later the bitcoin price goes up by 33% . The customer will receive 25 sats money-back. (25/75 = 33%)

This way, customers will not fear that they over spend if bitcoin always goes up. Even if bitcoin just goes up by 5%, it's still a nice feeling in customers' hearts that they receive some random sats from the business they spent at.

I want to implement this feature in the Pay-To-Join feature of my relay. I have no idea how to do it though LOL. 

 What about if bitcoin goes down? Should the customer send more sats to avoid under spending? 

 No. Then bitcoin goes down, it's just a discourt for customers. Business basically hedge for customers.