Why not buy one decent pen “shell” and then just buy the plastic tube with the tip and the ink afterwards?
I know many companies use pens for marketing but still, they could apply this too and also stand up for the environment while still do marketing.
That, and the ink can handle a bit of water unlike that of a fountain pen (unless you use India ink but then good luck if you have a leak and ruin your bag or shirt because nothing can get that stuff out)
PS The last time I used a check was in the 90s 😆 Do people still use them where you are? I couldn’t even get one from my bank if I wanted to.
There are a large number of water-resistant and water-proof inks; both Sailor and Montblanc make several. Legal documents are (usually) required to be signed in waterproof blue or black ink, so you’ll find a lot of selection in especially these brands that cater to business people.
Any ink that is indelible is going to be impossible to get out of clothing if you spill it; I think that’s rather self-evident?
I have used checks in the past couple years. Only like two or so, but enough that I somehow have a checkbook.
Google “waterproof fountain pen ink”. Here’s a comparison of 137 waterproof black inks.
India ink is for drawing, not for fountain pens. You need a specialised indiagraph to use it.
I’ve had multiple pens in my bag, the worst leak dirtied the nib a bit.