Shipping with TNT

[Impress51]

Hello - I am trying to link a customers TNT account with their Shopify website. We would like the prices from TNT to feed through to the site and be clear for customers. Many of the apps that are available seem to be portals which is not what we are looking for. The one app that I found that does Carrier-calculated shipping rates at checkout can only be installed once the site is live which will be too late! If anyone has suggestions they would be most welcome - thank you.