This issue could be made "black and white" (as it's apparently not) by the shop instructions stating that an outdoor heater is not the same as a patio heater and that no wording other than "outdoor heater" will be accepted. Period. Putting it in all caps, or bold, or whatever, doesn't have the same meaning.
Clearly, many shoppers who haven't done this shop were under the misconception that the terms are synonymous, and in the real world, to lay people, they are. Although apparently the OP was the only shopper of nearly 800 who asked for a "patio heater," other shoppers did indeed, per Dave's own statement, ask for the wrong thing.
Why not have the shop materials as clear as possible? To some, they might already be, but to others, they're not. An explicit, rather than implied, statement would solve the problem. You didn't ask for an outdoor heater, even though the guidelines stated unambiguously that was the ONLY acceptable phrasing? Shop rejected; no basis for dispute. End of story.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.