Peter, I tried to do the test with the parameters that you specified. Still everything works.
But I noticed the following on your order page, if you specify the quantity of item 3 (for 18 inches), then 1st Class Recorded (£6.50 inc vat). But if you specify the Qty of 4, then 1st Class Recorded will disappear from the list. It turns out that the weight adjustment of 300g for 18 inches.
Make sure the size attribute is filled correctly (weight adjustment).