Coding for max £ spent alongside weight -based shipping costs (without being on Advance plan?

Hi

Is it possible to add coding to overrule the Depending on Weight shipping costs, whereby shipping costs are set on weight BUT if a customer spends over £50, postage is free?

We have a customer who sells 5 products and there are 4-5 different weights for each, so it seems easiest to set a weight-based shipping rate and then it would be great to be able to add in some kind of coding that says any order over £50 is free shipping. He's only on the basic commerce plan and wouldn't be keen to pay the additional £13 per month for the Advanced plan, to be able to create a discount code, as he will be absorbing some of the postage costs anyway!

Thanks.