Taking payment on site
The cheapest way to get paid is to be paid before you leave. Every method that involves the customer doing something later is a method that competes with their evening, and evenings win more often than they should.
So the question is not really which payment method costs least. It is which one gets money moving while you are still standing there.
The three, on fees and speed
Bank transfer costs nothing and lands within seconds these days, but it depends entirely on the customer doing it. Standing in a hallway watching somebody find their banking app is awkward, and asking them to do it later is where the delay lives.
A card reader costs roughly 1.5 to 1.75 per cent depending on who you are with. On a £450 job at 1.75 per cent that is £7.88, leaving you £442.13. Money typically settles in a day or two. It also removes every excuse, because everybody has a card and nobody has to remember anything.
Cash costs nothing and settles instantly, and it carries the obvious constraint: it is income, it goes through the books like everything else, and the reason to take it is convenience rather than anything else.
What the card fee is actually buying
Look at that £7.88 next to the alternative it is competing with, which is not zero. It is the risk of not being paid today.
If taking cards moves a job from paid in three weeks to paid on the day, the fee bought you three weeks of your own money back, and it bought you not having to send two reminders and make a phone call, which is easily half an hour of your life per job that goes wrong.
On £20,000 of card takings a year the fees come to £350. That is a real cost and worth shopping around on. It is also less than one badly delayed invoice usually costs in time and financing.
Ask at the end of the job, not the end of the week
The mechanics matter less than the moment.
The best time to be paid is while the customer is looking at finished work and feeling pleased about it. That feeling is at its peak on the day and declines steadily afterwards, which is why an invoice sent on the Friday for work finished on the Monday does noticeably worse than one handed over at the door.
Say it plainly and without apology. "That is all done. It comes to £450, and I can take card now or send you the invoice, whichever is easier." Two options, both fine, no awkwardness. Most people take the card because it closes the thing.
What still has to happen afterwards
Being paid on site does not remove the invoice, which people get wrong.
The customer still needs a document showing what they paid for, and you still need a numbered record in your sequence. So the invoice is issued and marked paid, rather than not issued. The number still comes out of the sequence, because an unbroken sequence is the point of a sequence.
If you are registered for VAT, a card payment does not change the tax point in any helpful way, and the invoice still has to show everything a VAT invoice has to show. Being paid quickly is not a reason for the paperwork to be lighter.
Deposits and larger jobs
For anything with a deposit, take it the way that gets it taken. A card reader at the point of agreement is far more reliable than a bank transfer the customer promises to do that evening, and the deposit is the payment that matters most, because it is the one that buys the materials you are about to lay out for.
For larger balances, transfer is usually better than card, partly on fees and partly because some readers have per transaction limits. A £6,000 balance on a card at 1.75 per cent is £105, which is worth a bank transfer and a conversation about it.
Taking the deposit before you arrive
The deposit is the payment most worth automating, because it happens at the point where nobody is standing in a hallway.
A customer accepts a quote by email on Tuesday evening. If the next step is "I will send you my bank details", the deposit arrives on Thursday, or Sunday, or after a reminder. If the acceptance itself carries a way to pay, a meaningful share pay within minutes, because they are already at their desk with the decision fresh.
The same applies to the balance on a job you have finished and left. An invoice that carries a payment link is competing with somebody's evening; an invoice that requires them to open a banking app, type an account number and a sort code, and reference it correctly, is competing with their evening and losing.
None of this is about the fee. It is about how many separate actions stand between a customer deciding to pay you and having done it. Every one you remove is a day off the average.
What to carry
A reader, for the jobs where somebody wants to pay now and would otherwise have to remember. Bank details on the invoice, printed rather than promised, so the transfer route needs no further contact. And a way to issue and mark the invoice paid on the spot, because the version of this that fails is the one where you take the money and write the paperwork three weeks later, by which time you cannot remember which of the two Tuesdays it was.