It sounds like the text you sent, had something in it (picture, GIF, or some Emoji's for example ) that the message app has picked up as an MMS message, as at the point of sending the text, RCS was unavailable for whatever reason, and it has then sent it as an MMS message.
To avoid unwanted sending of MMS messaging, I have the MMS messaging setting switched off in the settings on my iPhone and also I switch off the Show Subject Field as well.
If the message had been sent over RCS, even sending a picture would not have incurred a charge. The only time you would incur a charge for RCS would be if you were paying for data use (but this would be the case if you were using any other app that used data).
You know if the recipient has RCS activated/switched on, if you go to write a text to them. If it shows Text Message RCS. If this is shown you won't incur a MMS message charge if you were to add a picture or something else that would incur a MMS charge if it was getting sent over SMS.
If it shows Text Message SMS, then they have not switched on/activated RCS messaging, and in that instance if you add something to that message (a picture, GIF or some Emoji's for example) then you would incur the MMS charge.
The first screenshot shows that the recipient has RCS activated/switched on:
This second screenshot shows that RCS is not available/is not switched on or activated on the recipient:
