"Is that all?" Kara - a small business might be sunk by that small amount 
So, have I got this right? Co-op decide things are a bit tight at the moment (and bear in mind they are a huge company) so they ask their suppliers, many of whom are small local businesses, to give them money to help them out, and if they don't, they'll withdraw the contract?
WTAF? Is this not playground bullying on a grand scale? Blackmail? 
Realistically, a small company may have to work their bollocks off for months to cover this amount of money, and then what will Co-op do in a years time? Ask for more?
Would there be a way to get together with other suppliers going through the same thing and stage a strike or some sort of legal action? (although, as I write this, I can imagine how unlikely that would be)
OP - unless there is a way to refuse Co-ops demands, I wonder if there would be a way to find other outlets, so you are not at the mercy of their bullying tactics.
Keep your chin up, these things will pass :)