If your shipment is stuck in customs, please call the courier and ask to talk to them about customs clearance. Tell them the issue you are having and give them the shipment tracking number. They will provide you with instructions. For Fedex or DHL shipments to the USA, this usually involves filling out and submitting a 5106 form to an agent ID email address that they will give you over the phone. For shipments outside the USA and Canada, procedures vary, ask the courier what you need to do.
For USA shipments, if you submit the 5106 customs form to us as directed in the order confirmation email, it is included in the shipment and there are usually no customs issues at all.