Hi Chad,
Case1: Actual Case
One option is total shipping price for each quantity.
1 – 12.99
2 – 18.9
3 – 31.85
4 – 37.8
etc..
You will have to configure same number of shipping rule in the shipping pro.
Case2: Same shipping cost for every 2 items
With the rounding option in shipping pro, following case can be managed with one shipping rule. Unit Price : 6.495 & Round : 2
1 – 12.99
2 – 12.99
3 – 25.98
4 – 25.98
5 – 38.97
6 – 38.97
Case3: Actual case for the first 4 quantity then per item cost there after:
1 – 12.99
2 – 18.9
3 – 31.85
4 – 37.8
5-999 – Base Cost: 37.8 Per Unit: 9.47
Refer shipping pro documentation http://support.wooforce.com/hc/en-us/categories/200853161-Shipping-Pro. Let us know if you need more details.