What is the most affordable UK postage company that integrates with Shopify?

[MegWilliams]

Hi there, 

I'm currently struggling with fulfilment.

What postage company is cheapest to use in the UK, and actually links with shopify?

I'm currently using the drop and go system at the post office but this doesn't speak to shopify, so unless you count every items shipping cost on the receipts daily and input them onto a spreadsheet there's no way to account for your actuals.

Has anyone compared pricings of shipment companies, and knows which app works best to talk to shopify?

Thank you in advance.