I hate that to.
I've ordered from them a few times. Especially their plant stuff... They have magnificently healthy plants and fair prices. They never ripped me off, and I even have ordered livestock all the way to Korea and it all came fast, cheaply and 100% healthy.
Just once I had my jar of zooplankton broken... Not a big deal.
They have this under their FAQ section:
Two charges on your credit card from us? All too often we are asked this question. Yes, our company charges separately for your product invoice and your shipping. We charge for shipping after your order is packed because these charges are based on the ACTUAL weight of your package, as we do not try to guess your shipping fee or base your shipping fees on the dollar amount you spend. Monday is our usual day to charge for product, and Tuesdays, Wednesdays and Thursdays are the days we charge for shipping. We charge FedEx list rates for the day your shipment goes out, as rates can vary from day to day.