Flat and USPS Shipping Method Combined- Possible?

[MAD King]

Hello,

I have a question for the experienced VMler her, but I am not even sure if this is the right forum for my problem.
I am using Joomla! 3.8.12 with VirtueMart 3.4.0 together with the plugins OPC from Rupostel and VM USPS plugin from Park Beach Systems.
I would like to do following:

I have a category with items which have all the same dimensions and weight. These items I would like to ship for a flat fee.
I also have other categories with different items and different measurements.

How can I set up the shipping calculation as following?

If someone orders only one item from the flat fee category, s/he gets charge the flat fee. This is simple.
Now, this customer orders two of the same items for the flat fee, gets calculated twice the freight.
More scenarios: Customer ordered these two items for the same flat fee, but another one from the non-flat-fee categories.

How can I tell VM to calculate per weight now and not do a sum of flat fee and weight based shipping?

Wow, I hope some can get through this what I wrote. 

Thanks

[MAD King]

Looks like there are no experienced Virtuemart people here.

[GJC Web Design]

what you are wanting to do is the classic problem of mixed shipping...

on their own the setup is straight forward .. but combine your two requirements and it immediately gets very complex

I have done setups like this .. my approach would be to extend the Parkbench plugin to do the manual calc of the flat rates and add it to the shipping discovered by USPS for the other items

But it isn't simple code and IMHO it certainly isn't configurable from existing plugins

[MAD King]

I was afraid that someone would say something like this. 
How do other user solve something like this?

[GJC Web Design]

they employ an experienced coder to code them a solution

This is such a one off request how could combination shipping methods possibly be in the core?

[MAD King]

This is such a one off request how could combination shipping methods possibly be in the core?

I do not know. I am not a developer. Just a little user without the knowledge of a developer.