Paypal checkout issue: Amount has changed

[Lewis24]

We have been experiencing quite a few issues with customers trying to pay for goods online before coming to collect them from the store.

We send them an invoice to pay to their email and then once paid, they can come and collect.
The constant issue we are getting from our customers  is "Paypal won't let me checkout as the amount has changed".
Does anyone know why our customers would be getting this message?

Thanks