I accidently paid for mine (minus the deposit) all in one. I got it from moviefigures.co.uk, which uses something called partial.y to manage the payments. I paid the deposit, then after a couple of weeks I thought I would pay a bit more, kind of like how you can freestyle, pay however so much you like, when you like, like how you can with Paypal Credit.
only it wasn't like that. I clicked the pay button, expecting to go through another step or two and then being able to pay the full amount, or pay a portion. instead, with NO warnings, it took the full remaining balance straight from my bank without so much as a confirmation. needless to say I wasn't happy about it. I contacted support and they said they couldn't reverse it, I could only cancel the transaction altogether (which would mean I would be refunded but also lose my deposit) so I declined. instead I asked a family member for a quick loan, as my account was now in the red (thankfully only by £80.)
Kind of a relief to have it all paid for though.