Shipping is almost reliable. If you pay more than $50 for a certain level of service you can expect shipping times reasonably close to what the shipper predicts. Inside the USA or between USA and Canada we have had no major delays. FedEx seems to be careful with that.
Overseas shipping is another problem. We estimate shipping delays based on what we have experienced in the past. Usually it's pretty close, depending again on the service level. Express Mail is pretty random though; we've seen three-day delivery and five-week delivery. Right now we use FedEx and DHL from China. DHL has the lowest probability of getting delayed, but we have to pay $250-$400 for a medium sized box of 15 to 30 pounds. EMS is a lot cheaper but the boxes come through mangled and late.