PayPal Totals not matching Opencart Totals

Hi Folks,

I need some help as i am not getting anywhere with the Opencart support team!

Basically, i have two Opencart stores running and have experienced the same error on both and was wondering if anyone has a solution or if they have experienced the same thing.

A customer has placed an order and completed checkout. When redirected to PayPal to pay (using PP Standard) The postage is no longer included. Customer then pays and is redirected to the site and presented with the order success page. Meanwhile, in the site admin, everything looks fine! The order is displayed as a new order and shows it as paid. So if i dont check PayPal, i have missed out on shipping costs. I have turned the advance error logs and this is what i got:
2014-09-12 9:24:22 - PP_STANDARD :: IPN RESPONSE: VERIFIED
2014-09-12 9:24:22 - PP_STANDARD :: RECEIVER EMAIL MISMATCH! ***********@sewsewfabrics.co.uk (obscured)

Research tells me that you are presented with this mismatch email if there is a problem somewhere - but doesnt specify where.

HELP!

Running version 1.5.6.4 and AJAX QUICK CHECKOUT V4.0.4 BY DREAMVENTION
www.sewsewfabrics.co.uk

Double check your settings in Paypal and make sure you don't have free shipping turned on it in. I don't remember where it is exactly, but I know Paypal has some options like that so that you can use their generated buy buttons if you want to.

I have checked the settings and it all looks correct... The strange thing is it doesnt happen on every order. There was someone who ordered something and that error occured. A week later she ordered again - again with the same error! She ordered today and it is fine. I asker her what device she used and she said originally it was a Kindle Fire HD and today was an iPad. Im wondering if it could be a device/browser issue?