When using the Express Checkout flow with PayPal Checkout the customer authorizes payment before knowing what the final shipping cost will be. If this final cost is substantially higher than the initial cost then this could cause problems when the order needs to be re-authorized for the higher amount, and there is a fix for this here...
https://www.ecommercetemplates.com/support/topic.asp?TOPIC_ID=115892

In order for the shopping cart to be able to give a reasonable estimate of shipping you really should have the shipping estimator enabled. Also, some shipping services need to know the state / county destination in order to estimate shipping and you can set a default for this (usually your home state / county) using this parameter...
defaultshipstate="CA"
$defaultshipstate="CA";
For the UK that would be for instance...
defaultshipstate="Kent"
$defaultshipstate="Kent";

Once set you should check that on the first checkout page you automatically get a shipping estimate without having to select a shipping state. It's a good idea to do this in a new browser window as shipping calculated in the estimator is cached.

Making sure that the Shipping Estimator automatically gives a shipping rate should reduce the need for re-authorization and improve the customer experience and sales as the customer doesn't need to log in twice to their account.

This is especially important for European stores (including the UK) or those that ship to Europe / the UK as the amount that can be charged above the initial authorization is much lower.

Vince

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