How do I combine shipping based on weight?

Can't seem to find this answer here! Any help would be appreciated!

I want my customers to able to combine their shipping based on the weight of the items added.

If they buy (1) item...I want to charge a minimum in shipping for that item to cover the initial cost of tare weight plus the item.

If they buy (2) items...I want them to be able to add just the weight of that item to their first item.....and so on (until they reach the maximum weight I can send per box).

How do I do this?

I won't be able to set a standard minimum base rate on all items since some are 1 lb and some are 50 pounds...so the base shipping rate minimum will change depending on which item they purchase.

ie: (1) item =1lb-Minimum $5.85 shipping PLUS weight of additional items
(1) item=50lbs-minimum $40.00 PLUS weight of additional items

....and so on

Can someone help? Thank you!
Windowla

ie: (1) item =1lb-Minimum $5.85 shipping PLUS weight of additional items
(1) item=50lbs-minimum $40.00 PLUS weight of additional items

... and what if I buy the 50# item and the 1# item?

the 50 lb item is a minimum weight by itself PLUS the ACTUAL weight of the 1lb item

Or the 1lb item by itself needs a minimum shipping rate PLUS the ACTUAL weight of the 50lb item (not an added minimum shipping rate on top of the added minimum shipping rate for item 2.)

...and vice versa

This is my problem!

What if someone just buys one? I need to cover that! But if they buy 2....I only want them to be able to add the actual shipping weight of the second item and NOT add on the minimum.

First, what country are you in and what is your preferred shipping method?

Second, are you sure this is the way you want to approach shipping? Let me give you an example. I'm in the U.S. and my only offered shipping method is our postal service. I have set a tare weight to cover the padded envelope I usually use, and a "handling charge" to cover the cost of the padded envelopes and the half-sheet-labels I print address and postage on. ZC figures the total weight of the order plus the tare weight, gets the postage amount from USPS, adds in the handling charge, and that is the shipping charge the customer pays. The amounts are small enough that I don't worry if the customer happens to place a large enough order that I use box packaging instead of the padded envelopes. And it doesn't matter if the customer orders a 50-lb item, a 1-lb item, or both; my costs and the shipping are both covered.

I went into gory detail because I suspect the "flat rate" you want to charge is to cover your handling costs as well as actual shipping, and I wanted to show you an alternate way of calculating that. If I misunderstood, feel free to tell me so.