Well I think it's obvious that a burglar with big bolt cutters can cut through any lock pretty much. There's only the lawn mower, some car tyres I've been meaning to sell for ages, very old tools and an old black and decker drill. Nothing I'm worried about getting stolen particularly, but that doesn't mean I'm going to leave the shed door unlocked and make it easier.
A deposit is a deposit, it doesn't really matter how much it is he's got it back and they've got the product back.
If it's more than the product they're less likely to run away with them... Think about it, he can get a pair off the bay for £20 but why wait for delivery if he can just spend £20 on a deposit and run off with them?
Did it take you the whole 3 weeks to get through the padlock?!