Amazon giving customers express shipping, saying it has to meet delivery guarantee date

Even when I am shipping an item the next day, sometimes Amazon upgrades their shipping and the cost is way more than they paid. For example, today they said I had to ship something express mail. The guarantee date was for Saturday the 11th. They put up express mail flat rate envelope for $24.90 and the customer only paid $4.98. First of all there is no way the item would go in a flat rate envelope. Secondly, the normal shipping method of first class mail package was showing it could get there by Saturday the 11th as promised. If that wasn't good enough, I found various other options on shipstation that would get there Friday the 10th (1 day before promised) that cost much less. For example; UPS Ground $8 Friday delivery, UPS 2nd Day Air $13.38 Friday delivery, Priority mail also about $8 and delivery Friday. Does Amazon just want to make sure I lose money on the deal? The notice that said I "have to ship express mail". Also said they would use the least expensive option to get the item there within the guaranteed delivery time. But that obviously isn't the case! I easily found 3 better options to get the items a day earlier than promised and the original method would be on time. Thank God I have a shipstation account. I was able to purchase the label for 1st class mail and just add the tracking to Amazon.