In any contract, as many variables as possible need to be fixed, else the contract becomes too vague and meaningless. Price and date can be easily fixed - the size of nucs is far more difficult to define - and their size changes with each passing day.
So - say you contract to supply 6 nucs for collection by (say ) 1st April for a deposit of $x, with the balance to be paid on collection. With that you then have something definite around which to work.
Ok - so let's say it's a late year, and the nucs are not quite up to an adequate size - and the agreed contract has therefore not been met. What can then be done ? I would suggest that the customer could be offered 3 alternatives:
a) an immediate refund of deposit.
b) a modest reduction in price (for the reduction in quality), with the nucs being accepted 'as is'.
c) an appropriate reduction in price (for the delay), with a new agreed date for collection.
Whatever choice the customer makes, the supplier will be taking the financial hit - which I think is only right and proper, as goodwill and protection of reputation are vital if a business is to prosper.
Personally, I prefer to sell on a first come, first served basis, with those showing prior interest being given a few days advanced notice of sale, with reservations being made accordingly. In the event of someone wanting to place a very large order, of course it might then become necessary to re-think this approach ... but I can't see this ever happening to Yours Truly.
LJ