Hi,
Thanks. Happy new year to you !
In the shipping formula field, you can use the tags {price}, {volume}, {weight} and {quantity}
The tag {price} corresponds to the subtotal amount.
However, you can't use conditions in there. You can only use normal operations.
For example, if you have the formula: {price}*0.1
it means that the shipping fee would be 10% of the subtotal amount.
So I don't think you can use the shipping formula in order to do what you want.
However, you can do what you want without this field.
Suppose you currently have one shipping method, with a shipping price of £10.
Then, you could enter 50 in its "maximum price" field.
Then, configure another shipping method with the same settings, but instead of a shipping price of £10, set it to £12, leave the "maximum price" field empty, and enter 50.001 in the "minimum price" field.
That way, on the checkout, if the subtotal amount is 50 or lower, the second shipping method will be filtered out and the only shipping method left will be the £10 shipping method which will be automatically selected. Similarly, if the subtotal amount is higher, the first shipping method will be filtered out and the only shipping method left will be the £12 shipping method, which will be automatically selected.