Yes, going to your customer as a source of operating capital is a time honored and proven way to raise the resource you need to grow and expand. Whether it is a gift card that collects the money upfront, or preselling cars before they are even manufactured -- bypassing the "money changers in the temple" has been part of business marketing and growth strategies forever.
If you stay within the context of "preselling" your product rather than offering either ownership or the perception of "ownership" you will be well within your rights.
If however you choose to use the words "invest", "investment club", "ownership" or anything else that in anyway seems to imply that the folks that are pre-purchasing your product are receiving *any*thing more than product-- then you run a very real risk of running afoul your countries securities laws. Don't do it.
If you would like some great examples of how other companies, organizations and individuals are using this type of direct appeal as a source of operating capital -- go and check out places like Kickstarter. (@Rob posted a great list in this answer)
What you will see on these other sites are some best practices in this model. Notice that they offer different tiers of access and benefits in exchange for increased amounts of initial capital.
Rather than send out a generic newsletter -- I would take the time with 550 customer to personalize them to how much money each would particularly save if they purchased 12 lbs of tea upfront. (Additionally figure out how much you will save in product sourcing costs and how you can pass this on as an additional value to club members)
Perhaps the basic "Pound of Tea" per month club is as you describe. But one level above include a tea tasting at a local establishment? Perhaps you can include for members a commitment to a monthly "tea lovers" newsletter for tea-ophiles. Perhaps you could include a special brewing pot for those that purchase the club membership before a specific date.