I am trying to set up my store's shipping rates so that if a customer were to purchase more than item, it adds an additional price to the shipping cost. The shipping price for one item to USA is $14 and I want an additional $4.50 added to the shipping price for every extra item ordered on top of that first one.
Thanks in advance!
This will depend on whether or not you want to use an app. There are a number of apps available that offer this kind of shipping rate customization (note, you have to have the 'calculated rates' option enabled on your store for most of them to work, and to enable it you either have to be on the Shopify plan, or pay $20 additional/month on lower plans, or be on annual billing for any plan.)
Else-wise, if you want to avoid an app, you would have to figure out a way to do this based on weight- or price-calculated shipping rates. Obviously, if there is a lot of weight and/or price variation between different products on your store, this would be a challenge.
For example, a price based rate (assuming the prices of your products are all $10):
Order total = $0 - $10: $14 shipping
Order total = $10.01 - $20: $18.50 shipping
Order total = $20.01 - $30: $23 shipping
and so on.....
On the other hand, a weight based rate (assuming the weights of your products are all 2 lbs.):
Total weight = 0 lbs - 2 lbs: $14 shipping
Total weight = 2.01 lbs - 4 lbs: $18.50 shipping
Total weight = 4.01 lbs - 6 lbs: $23 shipping
and so on...
Of course, if you're not using your product weights in any other way, you could just set all of them to the same weight and then use the weight based method.