I'm assuming you need to put things in storage quickly and only have this weekend available.(similarish situation going on here - just taking a coffe break break!)
See through plastic boxes help in that you can see contents, but they do come at a price. Black ones gets cheaper.
Plastic boxes are expensive, especially if needed fast over a weekend, (rather than bulk buy direct with next day delivery)
Most pound store ones degrade and become brittle and break very quickly because of the quality of plastic.Bigger pound store ones once full, are harder to move and often break in transport. You can be lucky, but generally it's a false economy.
Depending if the situation is long term use or short term, it may be better to go bigger/better and resell later
Known names will resell well- Really Useful, Strata, IKEA, hold value best as businesses buy them. (strata can be used outside)
but Whitefurze, Wham,.Crystal etc will also resell.
Best cheapest good quality deals are Ryman:
https://www.ryman.co.uk/office-supplies/storage-boxes
Next best but not as strong: Ikea: www.ikea.com/gb/en/cat/storage-boxes-10797/
I know you asked for plastic boxes, but may help with costs to know you can safely use clear plastic sacks in boxes for clothes/linen/ curtains/towels etc in storage, including long term.
If you need to be able to see contents; Banana boxes (Tesco's etc) stacked on their sides give you a 'transparent sack viewing panel.'
Labels and 'Sharpie' from Poundland.and sorry, no part of these things tend to be simple. Good luck and sympathies.