Individual plan non-fba account. Simple question about shipping charge

Hello.

I’m new to selling on Amazon and was just wanted to know why was $5.86 deducted from my sale order?

The buyer paid $4.99 for shipping but when I went to print the shipping label, I had to purchase shipping on my own and the only option was to buy a UPS label for $5.86.

Buyer paid $21.89 total. Product cost $15 Shipping was $4.99 Taxes/fees were $1.90 Amazon fees were $5.89 Total I got was $16 in the end which was then reduced to $10.14

So my question is why wasn’t the $5.86 label deducted from the $4.99 the buyer paid for shipping?