I only transferred mine the working day before collecting. Mine was was very last minute as well, got the text to say it was ready late on the Tuesday, I agreed collection on the Wednesday, they called to confirm everything on the Friday and I transferred the money same day, I collected the Monday.
I’d just give them a bell on the 27th and they’ll trigger the invoice if they have not done it by the time you call.
I didn’t need prof of transfer when I collected and me sending the money showed up within a couple of hours in my account. I did transfer the money in business hours though.