You'll have to decide when and how to round. Keeping individual billing items at high precision and rounding after summing them up can work; defining and documenting a rounding policy (or complying with whatever's legally required in your jurisdiction/domain) and rounding each individual billed item can as well.

Currency: USD Amount: 99500 Decimals: 5

Congrats m8 you invented floating point

Then you have no idea how floating point works. (Hint: its in the name)

You use 1/1000th or 1/10000th or whatever you need. You do not need “cents”.

How is the client going to pay that? They can't be charged half a cent.

Charge $0.995

Refund $1.00

Repeat

Charge $0.995

Charge Actual $1.00

Refund $1.000

Alternately

Charge $0.995

ERROR CHARGE AMOUNT MUST BE ROUNDED TO NEAREST CENT

In my scenario your payment gateway added a $0.005 fee. You told it $0.99.