PayPal - an incorrectly formatted shipping amount

[qme1ster]

Hi,

I have a custom shipping module written to present customers with 2 shipping options at checkout - standard delivery and collection instore. In order to encourage customers to collect instore we want to offer a negative shipping charge i.e. a discount for instore collection, but PayPal has a problem with these negative amounts it seems. The following is the error which is returned by PayPal after order confirmation at checkout:-

"The link you have used to enter the PayPal system contains an incorrectly formatted shipping amount."

Can anyone please suggest how we might be able to overcome this issue ?

Many thanks in advance.