As of late, I've been repackaging things into into straws. There are tricks to folding them to pack stuff into reusable containers for things like salt, but as an experiment I've been packing single doses of things like antibiotic ointment, sportsslick, or anything else that would otherwise be messy to repack by cutting straws to fit and then heat sealing the ends.
For single use applications even the smallest resealable containers are overkill and some liquids, gels and such are extremely messy to repackage, so instead I started heat sealing my own packages.
Here's an example of some gear oil I repackaged for gear maintenance single serve use:
To do it, simply use a pair of needle nose pliers to hold the straw closed with 1/8" of the straw sticking out on one side and apply a flame.
Insert your ingredients and repeat for other side. It heat seals nicely. It's practical for things that you can gauge how much you'll be needing in advance or that you can break up into needing several segments depending on need, such as sun screen.
Due to the collapsability of the straws its easy to get everything out down to the last drop.
Now I just need to test it out with water treatment in pre-measured amounts. It should safe weight by removing most of the container weight and still be air tight and it's pre-measured to just drop in.