Say my rectus end stop goes, there's absolutely nothing else I need to buy, in your opinion I should buy enough rectus fittings ( over £45 worth excluding vat) to avoid the delivery charge 
Just an example..
It's an interesting example because it helps to explain why I haven't paid delivery charges ever from Gardiners in just under five years.
If you don't carry a spare Rectus end stop in your van you're on a hiding to nothing. So, I carry spares. Always. Of everything that can fail. I keep them in a Quality Street tin in the van (not in the huge storeroom I have in my mansion). Then, as I use spares, the old ones go into another Quality Street tin. Every time I need a new brush or hose or whatever, I order a replacement for all the spares I've used. I pump up the order to £45 (if necessary) with ... Rectus end stops, because they are the part that goes most regularly.
Two benefits: I rarely have to stop working for the sake of a part and I never pay postage.
Not having spares in the van is a false economy anyway, as parts are going to fail and you're going to have to buy the replacement sometime, so all you're doing is putting back the purchase date.
Vin
PS. I admit the spare pump and the spare brush head don't fit in the QS tin but they don't quite fill the interior of the high roof, long wheelbase Mercedes Sprinter either; I still have room for my on board dining room with its cut glass chandeliers.
What a simple yet brilliant idea.
I carry spares but have to do an inventory from time to time before I place an order.
Gardiners always have something new to buy so £45.00 is easily achieved.
As regards the op one needs to compare postage costs with downtime. If you get stuck with a broken item and can't earn, a couple of days waiting for a delivery can cost you plenty. Delivery costs are nothing in comparison.
If you need that part, then pay the delivery charge and get the part as quickly as possible. You will 'recover' that cost very quickly once you start earning again. Sometimes you have to make occasional 'expensive' decisions or purchases by seeing the bigger picture.