When you are trying to check out and getting a "server errors", you might want to double check your credit card information you are inputting.
As our payment data on your account dashboard is connected to our payment gateway partner, authorize.net, any wrong billing address or credit card information will deny payment transaction.
First, you can try to check and see if it's a problem with incorrect billing address or payment information that is causing "server error" by inputting your credit card information on My dashboard-> My payment data.
After submitting all your payment information, and you get the green sign that gives you "Ok", your information has been verified with authorize.net and you should be able to check out.
Please make sure to also edit and add your shipping address to your account dashboard and click "Save", so you don't have to go through the same step again when you purchase next time.
Please note that pre-orders are not going to be included on your subtotal for your purchase, and if you are using any promo code, the promo code is only applied for the transaction of the incident only (which means you will not be able to apply promo code for the pre-order purchases or add-on purchases)
All promo codes are only used once, and you can not combine more than one promo code (or any other promotions, discounts or free shipping).