I did have the guy pop out and say thanks the first time. They were in second time so may not have noticed, though I would agree it'll get tiresome if they do it every time.
Before the recently departed neighbours started having gardeners, my predecessor used to get a bottle of wine off them when he did the lawn for them.
As I say I owe next door a lot of mowing but if it goes on for a long time being just me I might go for the cock and balls option.