Unfortunately, you will always find this type of idiot, and you just need to learn to protect yourself aginst them. Send him an invoice for the work, with a covering letter stating why his complaints aren't valid, and the if payment isn't received immediately, that you'll start a small claims procedure against him.
If you agreed or estimated that it'd take 1.5 days, and it then took 2, then that's down to you I'm afriad, but you'll get better at estimating the time required as you get more experience.
Here's a quick site that'll give you a few ideas on templates for the future, but you can do a search yourself for the sort of thing you're after. I assume that you have a computer and a printer; you just need to learn how to use the word processing software, and create templates that you can use time and again.
Always makes sure you get their full name, and then put the quote on paper and get them to sign it; that'll make sure they know you mean business. If they won't sign it, then walk away. Take pictures of the site before you start work, and then after you've finished the job to back up any disputes, and don't be afraid to let the client know what you are doing. The more professional you appear, the stronger you're position will be. Always get the money for materials up front, and be quite blunt about it (albeit in a joking manner) so that if you get stitched up, it's only labour that you've lost. You did walk on the plants, and you either need to warn future customers that it's their responsibility to remove or be prepared to replace plants that are in a vulnerable position, and the guy should should know that, so he'sjust trying to wriggle out of the money. Maybe you could offer him a nominal reduction of £5 to replace the plants as a gesture of goodwill, and on the basisi that you should have advised him of the dangers of getting damaged beforehand.
As you're obviously starting out, you could ask customers to source the materials, and you could go with them, but obviously, you personally couldn't use their debit/credit card. But part of you're profit will come from buying materials at cost prices, and charging retail. It's not a lot, but it all helpsa, although it'ssometimes more hassle than it's worth - your choice on this one.
You do still own the materials till he's paid for them, but you've got to be able to get them back without damaging anything, so as long as it's panels that can be lifted out cleanly, then there may not be a lot you can do.