Let's change the question: how to offer free carriage for orders over £100?
It's easy. All you have to do is create a shipping option using a formula based on the amount of the order. The following example is based on the order's VAT inclusive price.
We'll define a carriage fee of £5 for an order of less than £50, £4.50 for an order between £50 and £99, and carriage free for orders greater than or equal to £100 VAT inc.
To do this, click on Commerce > Shipping Options > Add.
Give your option a name then click on the Calculation Formula tab.
In the Type of Formula field, select Tiers based on the order total VAT incl.
Fill in the remaining fields as in the following screenshot:
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If you wish to offer free shipping according to the order's amount AND weight, we suggest reading the following online help document:
How to Offer Free Shipping According to Order Amount and Weight?