Then refund the customer's credit card for the difference and don't swap it for a different product, or allow the order to go through with quantity you don't have.
I ordered '10 Federal 22 Long Rifle 40gr Solid Champion AutoMatch Ammunition 325rds - AM22' (33625) and two 'American Eagle 5.56mm 55gr FMJ Ammunition 150rds - XM193BK150' (82265) but they only shipped one XM193BK150, I logged into the website and now the order shows only one XM193BK150 and they modified my order to include 8 boxes of American Eagle 223 55gr FMJ Ammunition 20rds - AE223JLC (NOT XM193) at a cost of $60 more than my initial order (that includes the two boxes of XM193BK150).
So far my card has not been charged the difference (nor credited for the missing box of XM193BK150) and I am trying to get a hold of customer service with no such luck so far in hopes that they don't ship out the .223. I think it's really bad form to modify a customer's order without their consent, and causing a lot of confusion in the process.