f you want to to omit decimals in product prices, then you should:
open the "Settings → General → Formats and Units" page in your control panel;
enable the "Hide trailing zeros in fractional part" option
After that all trailing zeros in fractional part will be removed. So for example $40,00 will be displayed as $40
If a product price/order total has fractional part/decimals (e.g. $40,57) then it will be always displayed to show the exact cost to customer.
Please note, that the fractional part is removed automatically, if your currency doesn't have it at all. For example, Yen. If you use such a currency, then it isn't necessary to enable the "Hide trailing zeros in fractional part" option.