Here is my tuppence worth...
Yes, there is a different business model other than "per unit", but everything depends upon your target customer.
I do PAT testing (ugghh!)
PA Testing...
ISITEE, the correct term...but I accept that most of MY target group will understand PAT Testing, so that label does have a place, despite its inaccuracy.
My target group is private landlords and small, local offices and businesses. This dictates my business model, as does the fact that I do not have to make a living out of it.
I get work from local estate agents who deal with rental properties. They have loads of "electricians" who do this stuff for them, because the tenancy agreement requires some form of certification that appropriate checks have been carried out to show that appliances in a rental property are safe for use...and most are happy that a "PAT test schedule" satisfies that requirement. ( I would mention that I am in Scotland, so there are different requirements here for the PRS-private rental sector).
My model is a simple charge for the inspection and testing, regardless of number of items. Why?
Because very few rentals have more than 20 items, some have 6, some have 30, but a fixed price quote evens everything out over time, and provides security of a fixed price for the landlord.
As for small businesses and offices, I usually have a quick look-see first, but still use a fixed price structure. Like my price? I'll do the job. Don't like? No problem, get a sticker jockey in. Those who are only concerned with price are those who are less concerned with safety and only want a piece of paper...these people are not my target.
My target group don't need/want digitally produced inventories, automatic emails, all that clever stuff...they want a list of tested items and a pass or fail. Anything more, I am likely to decline. There are many reasons for this, but that's a whole different topic.
Thus, as I said earlier, it's a question of choosing your customer group, and sticking to it. They know what you provide, you deliver, they pay, and job done. If you try to do absolutely all types of customers' works, you will end up with stuff you don't want. if you stick to your niche, all goes well and you sleep at night knowing you have done the right job.
Two points i would add:
1. If you won't test "fixed" items, you may find yourself out on a limb. For example, let's say you are testing a B&B. In bedroom 1 there is a wall-mounted heater, plugged into a socket. Easy job, plug it in, pass or fail. However, in bedroom 2 is an identical heater, but wired into a FCU. Do you refuse to test that one? In my view, you ought to have both the equipment and the knowledge to test that one too, else you are not providing the service that the customer requires. Nobody wants a list of stuff that is marked "Could not test". That is not really of much use to the often inexperienced customer...after all, he is hoping you will be his "one-stop" shop for all his "appliances". You won't get repeat business from him, or the introducer, because you didn't test stuff.
2. There can often be items that need replacement. For example, on FVI you find a 13A fuse in an item that should have a 3A or 5A fuse (that's another story, ok?). Do you charge for that replacement fuse? Of course not...not on a fixed price structure.
Does an ancient appliance with a "rogue plug" on it get failed because of that? No, of course not...not on a fixed price structure. If the L and N pins have no insulation, you put on a new plug...end of. Then, when you leave the venue, you have inspected, tested and fixed where appropriate, and the customer is happy. No need for further work, no need for disclaimers...writing a disclaimer takes more time that changing a plug or fuse, and a customer wants his job done and dusted. By all means write a disclaimer for matters where there really is no option, but don't fudge the issue because of a minor remedial matter which can be taken care of for a few pence.
The OP asked if there is another business model apart from "per item". I submit that there is. The above works for me, but I fully accept it will not work for everyone.
However, my attitude to risk, and my view of what is a proper service, means that I am always confident that the job I have done is complete, accurate and safe, for a fair price, with no comeback...and plenty of repeat business.
If you want to make a living out of ISITEE, good luck!
If you want a sideline which can lead to other work, go for it, do a proper job, and reap the longer term benefits.