I'm well versed in this kind of thing but it depends what & how you're ordering, most custom checks happen at international borders so if you ordered an illegal drug and it was shipped from what's referred to as a "hot country" such as the Netherlands, in standard mail package it has about 60% chance of getting through depending on stealth used, if the shipper uses good stealth such as a decoy or false bottom then it's got like a 90% chance of getting through.
See customs don't really open packages, international customs X-rays which is only useful for firearms and sometimes grabs a bunch of packs coming through and throws them all on the floor and then run sniffer dogs over them, that's the entire check. There's no database to log who did what so even if something gets caught any small change in the details will appear like a totally new package like any other pack to customs.
One time a gram of H I ordered from NL got caught and I just got a seizure letter with no further action but I know another guy who ordered half an ounce of H and they kicked his door in, questioned him for a day and then let him go cause no evidence.
Your average copper has no idea & no training on stealth shipping or cryptocurrency and there's nothing they can really do apart from try to get you to confess.
If you're ordering with proper OPSEC (from the darknet using PGP encryption) domestic to a PO box, there are 0 checks, the only way that's going to get caught is if it either stinks and a postie reports it or the supplier is already under investigation & surveillance.
If you're ordering off the clearnet for example SN from Ebay then you might get a visit from da po po regardless of customs checks.
Hope that helps.
