Trouble with combine shipping

I sell 2 different type of items on my store so since I have 2 different prices for the items so I had to use per product module....Now the problem I am having is how do I combine shipping...

Let say the same customer is buying from the store more than one item and the first item shipping is $4.00, but then they are buying another item, I don't want them to be charged another $4.00...I want the shipping to be cheaper when it's combine...

What should I do? I looked every where and don't know how to do the combine shipping..I am in the USA...Please help...

Is there something unique to the 2 products that would distinguish them from one another?

Yes, one product I sell is T-Shirt which run for $12.99 and I also sell stickers vinyl decal that run for about $5.00....

But is there something unique about the data in the database?

Do they have a pattern to the products_model?

Do they all belong to a particular categories_id?

Is there something to say specifically if checked or referenced in the products or products_description table what kind of product they are?

Otherwise, you could add a new field ... products_shipping and have a given setting or value for this ...

Then customize the item.php Shipping Module to charge 2 different rates for the number of items in the cart for each value ...

Takes a bit of work, but it can be done ...

This is my situation...

I sell t-shirts and decals...Both item have different prices and different shipping cost...For the t-shirt I am charging $4.00 for shipping and for the stickers $1.00 for shipping....

So the company that I bought the zen cart from added in the module a shipping box so I can add shipping for each item...I went to Catalog then I clicked under products/category, then I clicked add new product and when I put my item information like how much is cost, the name of the item, product etc, there is a place there for shipping too...

Now my problem is combine shipping....Don't know how to do that...Let say the same customer makes more than one order, so the shipping would be $4.00 for first t-shirt and the second I want it to $1.50 for each additional item when the same customer buys more than one item, this is where I am having the problem....I don't want the same customer who buys more than one item be charged $4.00 for first item and then addiotional item again $4.00...I want to be less...The same goes with the stickers, instead of being charged for each additional item $1.00 I want it to be less...

I looked everywhere and I can't find anyway solution to this....I try using on shippng module per item, but it will only do it if I have one shipping cost and since I have 2 different cost it will not do it...

I have version 1.2.7 of zen-cart and I don't know if I might need to upgrade that version to the new version...And if this will help...

Let me know what should I do? I been going crazy for the past 3 weeks with the same problem...