If you need to add surcharges, you may consider one of the options below:
1. Add a surcharge in the Shipping Table Rates extension. It will be necessary to create additional rates for this purpose.
For example, if you have a general rate in your method, you can create an additional rate for your specific conditions (shipping country, weight, etc.) with a surcharge already applied, and then this rate will be used as soon as the conditions are met. Here, $2 surcharges are added for particular item quantity ranges:
Here's a comparison of when the base rate is applied on the storefront:
And when the surcharged rate is applied:
2. As a more convenient way, we can offer using our Shipping Rules extension. It works perfectly in combination with Shipping Table Rates, and you won't need to create additional rates in Shipping Table Rates configuration, instead you'll have one base rate and as many shipping rules as you need, and they'll overwrite the base rate.
To create a shipping rule, go to Sales > Shipping Rules and add a new rule. In the Shipping Methods tab, specify the name for the rule in Rule Name field and choose the shipping carrier/method that should be affected by the rule in the Shipping Carriers and Methods field:
In the Rates tab, choose the rates calculation type (here Surcharge is selected as the shipping rate needs to be increased) and specify the amount of the surcharge:
Finally, in the Conditions tab, define the conditions under which the rule needs to be applied:
The result on the storefront will be the same as for the first option: