You can specify USD, EUR, BTC or Sats currencies for the blog post. If you set the amount in a FIAT currency like USD or EUR, the BTCPay server will convert the amount into BTC or Sats. Depending on the settings you have made in the BTCPay Store.
The amount the user pays can range from 1 cent to several euros. Thanks to Bitcoin and Lightning, amounts can even be settled in the millicent range without any problems.