A standard SMS reminder is up to 160 characters long and costs 1 credit (credits cost as little as $0.15 when you buy in bulk). Reminders that are longer than 160 characters will cost more than one Halaxy credit in accordance with standard SMS rates (e.g. if your reminder text stays below 305 characters, two credits are used). If you select two-way reminders, there is no cost for patient responses to the reminder.
When you are writing your reminders, there is a preview which provides an estimate of how many characters you have used so you know how many credits each reminder will use. Please be aware that when dynamic terms are expanded, this may increase the number of characters.