Is it possible to disable an active shipping module for certain items?

The shipping price of my items is based on what country it needs to be shipped to as well as weight. These 2 criteria are covered by the shipping modules I'm using. However, the shipping price also depends on wether the items can be shipped as letter mail, or as a package. None of the modules I found is able to do that. That means I have to use more than one module to obtain the right shipping price.

That in itself is not a problem. But as far as I know from both working with Zen-Cart and from what I've read on this forum, in Zen-Cart it isn't possible to make a shipping module active for only certain items. This means I either have to charge letter mail fees (and charge not enough for the items that need to be sent as packages), or charge package mail fees (and heavily overcharge the items that can be sent as letter mail).

What I'd like to know is wether what I've found on not being able to use a shipping module only for selected items is correct, and also if anyone knows a simple solution for this problem. I've read some posts on this subject on this forum, and already found the suggestion that I could add an extra variable to a certain selection of items and rule out one of the shipping options based on this variable. However, I'm not a star in manually adding variables to a database, and even less in changing the checkout page so that it drops one of the shipping modules. So any help on how to do this in a simple way is welcome!

You can set all kinds of criteria or rules for shipping modules ...

But, you have to determin what is the alternative when such rules are set ...

Does something turn off?

Does something turn on?

Does something get altered or increased in price?

Well basically this is the situation:

I have items that can be shipped as lettermail, but also bigger items that need to be shipped as parcels. Also, there's a difference between national and international shipping. So this sums up to four different kind of shipping types:

National lettermail
International lettermail
National parcelmail
International parcelmail

The parcelmail for both national and international are covered by the TPG Post shipping module that I downloaded from this site. It checks weight and destination, and based on that checks the current shipping price from the site of the shipping company.

Besides this module, I've installed another module from this site, the National and International per item shipping module. This module has a per item flat fee for letter mail to both Holland and the rest of the world. If people buy an item that can be shipped both as letter mail and parcelmail, both options can be chosen.

However, for larger items, the per item shipping module shouldn't be an option because these items have to be shipped as a parcel. So basically I want to turn this shipping option off, not based on weight, price, or destination, but based on size. So far I haven't found a way to do this. If there is a way to do this, I'd very much like to hear about it! Thanks in advance for any help :)