I hear farmers complain about this every year. That is why I use multi-tiered pricing. Full shares are $550 but shareholders get substantial discounts for early payment. $450 in January, $475 in February, $500 in March, $525 in April, full price of $550 in May or after. $300 for half shares anytime right up to start of the season in June.
Either you are going to have to make it worthwhile to get money in early or you won't get it. You may think your shareholders have your best interests at heart, but in reality you are waaayyyyy down on their list of priorities.