It looks a bit weird and isn't the commonly used term, but is it not correct? The apostrophe would be marking the shortening of "terms" to "t" and "conditions" to "c", of course.
It looks a bit weird and isn't the commonly used term
It's certainly a commonly used term, and a search finds plenty using each of the variants "Ts and Cs", "Ts & Cs", "T's and C's" and "T's & C's.
Whether one uses apostrophes with single-letter capital abbreviations is something different style guides differ on. If you aren't writing to a style-guide then I'd note that to not use the apostrophe is generally the more modern style, and being consistent one way or the other is a very good idea.