Error processing IPN message for postal orders with a shipping fee

Hello,
I receive errors whenever tickets are purchased, and the customer has selected the postal delivery option, and where there is a shipping fee. Here is an example

PayPal receiver email did not match account or payment currency did not match payment on 11214
Array
(
[transaction_id] => 3Y375378S9528800L
[price] => 41.00
[currency] => GBP
[email] => [email protected]
[first_name] => Joe
[last_name] => Blogs
[fee] => 0.77
[parent] =>
[status] => Completed
[purchase_id] => 11214
[shipping] => Array
(
[street] => xxxx
[street2] =>
[city] => xxxx
[state] =>
[country] => GB
[code] => XXXX XXX
)

)

The customer purchased 4 tickets at ￡10 each and paid ￡1 shipping fee. The cost on PayPal is ￡41 and this is the price returned in the IPN message.
The cost displayed in the Payments Post is just the ticket cost of ￡40. I believe that the IPN validation may not be taking account of the shipping cost and this message is being triggered in error.
This IPN message was sent as the result of a ticket sale and not another product in my store, and I do not receive these error messages when a customer chooses to print the tickets.
I'm using version 1.6.0 with WordPress 4.9
Many thanks for your time
Dean