Dear Jenny,
You have given a timely warning of the perils of oversize on the new postal conditions for letters. Although the maximum weight for a letter has been raised from 60 to 100 grams, the size of an envelope to go at the minimum rate is all important (240x165mm). The most practical largest sized envelope to use is a C5 (229x162mm). This would allow A5 sized puzzles to be returned flat (or if folded once in a standard 220x110mm envelope).
I suspect your correspondent was within the weight limit with regard to content but had used an old sized envelope beyond the present limits; hence it would be treated as a large letter requiring a 40p second class stamp instead of a 24p one no doubt used.