I bought a rack of 2 shelves from Amazon and it bended with just 5kg on the top shelf
What are your storage solutions?
I have buckets from the hardware store…
Perfect size for rolls and no issues stacking them 5 buckets high.
4 to 5 rolls per bucket depending on you desiccant preference.
As for a shelf starting to bend>
Screw a support bar under it…
Some cheap timber board of 12 or 15mm thickness will do fine.
Add some glue put the screw through the shelving board…
The strip does not even have to very tall - 10cm is usually plenty.
You can also use angle brackets from aluminium or steel if you happen to have them around for free - anything that won’t bend easy…