Something odd about UK shipping charges (combined / multiple deliveries)?

[Andyr]

I've got an order (to the UK) ready to place, but something seems to be up with the shipping. It's for a handful of magazines, including the final Dragon (which is on preorder status).

If I choose to despatch the currently-available magazines now and the final Dragon when it comes out (i.e. 2 separate deliveries), I can choose between the following options:

For the currently-available magazines:
-USPS Package Service ($6.86)
-USPS First Class Mail ($32.58)
-USPS Priority Mail ($35.10)

For the final dragon:
-USPS Package Service ($6.86)
-USPS First Class Mail ($12.42)
-USPS Priority Mail ($22.50)

This all seems reasonable, and means I could get a total postage of $13.72 for my separate deliveries.

If, however, I opt to wait and have all the items sent together then I only get the USPS Priority Mail option (quite pricey, at $39.30) even though the website suggests that waiting is cheaper. Only the one option comes up. Is there some reason I cannot just get the USPS Package Service for the combined delivery, as I am quite happy to save ~$30 and wait an extra week or two? Or is this a site problem? The pricings seem somewhat counterintuitive.

[Gary Teter]

There's a maximum weight for USPS package service, and combining all the issues together pushes it over the limit.

Normally "Ship Together" is cheaper, so the website automatically says that. But obviously in this case that's not true.

[Andyr]

Ah, ok--I'll go for multiple deliveries then. :)