Shipping modules and config

I am very new to Zen-Cart and have been working with it extensively now for several weeks. I have my store setup the way i want it and have a question on the shipping modules.

I am using the USPS shipping module, how do i get a product to only show one shipping method or tie a specific product to a shipping method?

An example would be: I have four products A, B, C, and D. Product A are small items and can fit in the small flat rate envelope and product B is larger and will fit in the small flat rate box. product C is larger that B and will fit in the medium flat rate box. Sometimes I will have larger light weight products. If a customer were to purchase Product B then the shipping options that are shown will be for the envelope and small flat rate boxes. I did notice that i could adjust the Minimum Weight (lbs) and Maximum Weight (lbs): fields and correct a few issues. But if its a large item that weighs only 1 pound and I have to pack it in the large flat rate box then the shipping options available will be for the small and medium flat rate boxes.

Is there a way to have product A always ship with flat rate small box and product B ships with the medium flat rat box?

Maybe I am going about this the wrong way.

Any ideas on configuring shipping would be most appreciated.

I'm sure there is a coding way to do that. Ajeh is the goto guru. We choose to stay within the 'stock' USPS module to minimize issues when the USPS implements an update twice a year.

If I understand the issue, we use weights to filter the FlatRate box sizes.

We have many products which we can fit a nearly unlimited number into a small flatrate box.
We have a few products which require either a Regional A or Medium flatrate box.
We have one or two items which require a Large FR box.

We set the weight range of the small flatrate boxes 0-1.5 pounds
Regional A 0-4 pounds
We set the weight range of the medium flatrate boxes 0-7 pounds
Large FR 0-70 pounds
That scheme also works for first class thick envelopes for the small items that weigh less than a troy ounce each.

Most product weights are set to 0.1 pound.

Set the items which require a Regional A box to weigh greater than 1.5 pounds.

Similarly, the Med FR items weights are set to greater than 4 pounds
Large FR items weight set to greater than 7 pounds.

That way, the lesser flatrate boxes are removed from display to the customer if the combined weight will not fit, including the TARE weight settings.
If a customer chooses a larger flat rate box for small items, well, I'm sorry that they chose to select a more expensive packaging.

Will this work in your case?

It makes sense and I will give that a try and test the setting you described in my test environment.

Thank you