I had an idea about how to handle the hashpacks to make the system simpler and faster for everyone. Essentially you buy the machines in as much as you need to fulfill orders for now however moving forward I suggest this plan. Only purchase units that produce 50GH (or more if possible), instead of selling people their own personal units just sell them a percentage of the shares generated, proportional to how much they payed for their hashpack IE
I buy a 5GH, instead of you buying specifically just 1 5GH and handling that, just sell me 5GH worth of shares from the 50GH. If there are any GH left over you keep them for the pool to pay off electricity and buying more hashpacks. This way you can handle not dealing with the long shipping times and still supply people with what they payed for. Furthermore if you buy larger supplies of 50GH (or whatever is max) you will save on money per gigahash and space and electricity thus making it more profitable, efficient and faster for everyone involved.
Now obviously if someone orders a 50GH for themselves this is less profitable for you as a pool in one sense as you can't split it out to a variety of people and you need to order a whole nother unit for them, however you could allocate left over space from various other units IE the shares for those units. Actually if you are just selling the shares then you can just sell anything in between 10MH-50GH as you aren't giving them the literal device which makes for more efficient management of units. You can effectively just count all of the shares generated by your units, calculate how much GH went into making it, lets say per 6-24 hours then pay out to everyone who bought a hashpack based on the GH they purchased sort of like a stock rather than purchasing an actual unit. Again this provides for much more efficient handling of the units as you can buy in bulk, split it into much smaller sub divisions and make the most efficient usage of your hashrate/shares produced. You can also allow people to sell back their share of the hashrate which would be based on how you wanted to handle that personally. Either way I would love to see you implement the system I devised above.
Edit: One more note, for the sake of sustaining your pool I suggest you shift from PPS for the hashpacks to
Eligius's Capped PPS with Recent Backpay in order to never accidentally drive yourself out of business by having to pay the overhead of 100's of GH of PPS which don't necessarily guarantee a block as fast as you would like, you can then lower the fee down to like 1-2% as you don't need the fee to cover potentially bad rounds.