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just burn the source tree onto a dvd and tell them to keep it in a safe place
(really just the source tree and nothing else: no notes, no list of required etc... can figure that out themselves if they want to self/get someone else to work on it.)
Most of my customers are long-time customers so I have VPN access to their net. That means that when I work remotely it is via Remote Desktop on the customer's machines and so the work I do never leaves the customer site.
For other clients I have shared end products in a folder on my own web server (password-protected if confidential) or uploaded to customer's sharing platform of choice (lots of cloud offerings for file sharing).
Been a long time since I used CD-ROMs and pen drives - but yes, that's a rock solid solution. Go with the pen drive if you have to ship physically; optical drives are becoming scarce.
CD and/or USB stick, with PC source code & build info + exes and PLC code (where appropriate). Also including circuit diagrams of hardware (where appropriate) + User and Maintenance manuals (as appropriate) + config files and simple souls setup.
Install it on their server. Part of the deal almost always includes installation; otherwise there will inevitably be some environmental / configuration issues and it always takes more time to talk through what the problem might be than to just resolve it. In most cases my clients are not technical and if I sent them a ZIP file they'd not be able to open it, let alone install it!
95% of my projects are web-based so "installing" can be anything from copying over a new aspx.cs file to setting up a new IIS website and fully configuring it.
Very occasionally a project is just to build a DLL (and include technical and user documentation) in which case I'll usually just email the deliverable as one .zip file, with a separate one for the source code and any additional documentation / notes.
I cycle a lot too, rain doesn't bother me apart from making things a bit more slippery... Singapore ... rain is actually refreshing.
Lightning though, well adds excitement. Tall trees and lightning, perhaps not.
Seems most Asians are scared of the rain, few drops and they run away. I tell the kids it's because Asians melt in the rain, but being only half Asian they've got my waterproof genes.
A common question I get from the Asians (re cycling) is "what happens when it rains."
My answer: "I get wet." Usually shuts them up.