Can you post a screenshot of this message that states this "will take you over the spend cap"?
Android phones (I assume Apple do something similar) have an in-built warning when sending texts to shortcodes, that the message may be chargeable. This is to cover the fact that text message bundles only generally include SMS to other mobiles, and texts to shortcode numbers are nearly always chargeable at a higher rate.
Your phone has no way of knowing what an individual shortcode is charged at, much less that "7726" is a free one. Hence the on-screen warning that I suspect you're referring to, will be totally normal but also perfectly safe to dismiss. Let's see it if you're not sure.
