Sadras
Legend
This to me would ruin all the mystery of a who-dunnit like this. No thanks. The shopkeeper should be a primary suspect. The reason to ask for an insight check is to maintain that air of mystery and doubt. The shopkeeper is less likely to be involved, but there's no way to be certain.
What air of mystery would that be?
1. The one where the PC knows he rolled well or badly or just plain average - leaving everyone else in the party make Insight check rolls too?
2. The one where the DM narrates that the shopkeeper shifts uncomfortably in his seat as questions are asked?
Firstly do you really need an Insight skill to notice that? Secondly, any suspicious descriptor now added by the DM about the shopkeeper means the PCs will press on with diplomacy, bribery and intimidation - and usually in that order.