Need shipping module to limit shipping options based on category or product

Okay...

Now, what file would I do that in? Would it be in the USPS file:
includes/modules/shipping/usps.php ?

I'm not all too adept with php coding, so how similar would it be to this post:

http://www.zen-cart.com/forum/showthread.php?t=131665&highlight=shipping+limit+based+product+category

Sorry for being so dense... =P

Regards,

~Rachel
http://www.colema.com/store/retail

I've been looking over the thread that I referenced in my previous post, but because it is for UPS and I need to make USPS unavailable for these specific products instead, I don't know exactly how to tweak and implement the coding appropriately.

In short...please help! =)

I am having a similar problem, but I think a little less difficult as the original post.

I have item "A" weighing 27lbs and is already boxed and would ship as is.

I have item "B" weighing 1 lb and is not already boxed and could ship combined with other items not already boxed.

However, if a customer orders both items "A" and "B" the ups shipping module automatically assumes total weigh 28lbs and one box. When I actually go to ship this I have two boxes, one at 27lbs and one at 1 lb. I get :censored: on the shipping cost because it costs more to ship two boxes at 27lbs and 1 lb than I would one box 28lbs.

Is there any way to let zencart know that item "A" cannot have combined shipping with any other item?

Thanks in advance for any insight or assistance.
:lookaroun