RCS messages are sent using the same credits as SMS. The number of credits used depends on the fallback SMS (see the FAQ above for details). If, when creating your fallback SMS, you exceed the number of characters allowed for a single SMS (160 characters), the SMS will count as 2 credits instead of 1, so your RCS will cost 2 credits. To make your RCS cost only 1 credit, you will need to reduce the number of characters in the fallback SMS.