When I send a messsage to a short code I am told:
Calls or texts to short codes are chargeable and are not included in your standard allowance or Add-ons. See our call charges page: https://www.idmobile.co.uk/help-and-advice/call-charges
The linked Call charges & Add-On pages allows me to determine that messages to the short code I am using will cost me £0.15. However, I can’t see how to add this to my plan, on which I have a £10 cap. There is no option under Add-ons.
It is odd making it so difficult to increase my spending.