Increase shipping cost based on quantity

Hello,

I am looking for a shipping solution that increases cost based on quantity. We currently have flat rate shipping which works fine, but now we are selling bulk items and need to charge more. For example:

If someone buys up to 20 of item A, the shipping cost is standard, but if they purchase more than this then an additional ￡3 is applied. I have attempted to use table rate plugins but none of them seem to offer this feature without the premium upgrade.

Does anyone know of a way to achieve this?

Thanks in advance!