With regard to Round Up and a LOT of other garden liquids, it kind of frosts my behind the amount of liquid you cannot get to pump out or spray from their containers. In most cases, you can't get the top off to combine what's left into another container.
I have never used the pre-mix, nor the item Cab showed.
Neither fit my needs.
Early spring, I buy RU in 1/2 gallon concentrate.
If needed, I'll buy a 1 quart RU concentrate if I use the 1/2 gallon.
Once I bought a 2.5 gal Tractor Supply generic version of RU.
While it did work well, took a week or so before I saw effects.
I've had a 1 quart ground clear concentrate for a few years now.
Each has it's own 2 gallon garden pump sprayer.
I also have same size sprayer for Sevin.
A 2 gal pump for Dawn soap and water to kill box elder bugs outside.
Watch for sales, ACE hardware had Hudson sprayers for $8.00.
All concentrates, along w/spray paint cans and such, go in a plastic milk crate, normally stored in my barn, over winter in basement.