This is actually the big problem with PayPal Express, you need to use a default shipping method and cost.
We will investigate to find a better solution, but now we are limited...
To answer your question, you can find this code in OrderSummary.cshtml :
<divclass="addon-buttons">
@*Payment method buttons (e.g. GoogleCheckoutButton, Paypal Express)*@
@if (Model.ButtonPaymentMethodActionNames != null&& Model.ButtonPaymentMethodActionNames.Count > 0)
{
for (int i = 0; i < Model.ButtonPaymentMethodActionNames.Count; i++)
{
@Html.Action(Model.ButtonPaymentMethodActionNames[i], Model.ButtonPaymentMethodControllerNames[i], Model.ButtonPaymentMethodRouteValues[i])
}
}
</div>
The code is conditionned by @if (Model.IsEditable)
If you move the code out of this if statment, PayPal Express button should appear everywhere in the chechout process. It could be a first step to see if it's efficient to select shipping methods before calling paypal express