Customs can still be an issue on dark net markets depending on which vendors you order from. More reputable vendors will generally apply better stealth tactics than businesses from clear net markets, because they know the stuff they're handling can be illegal in certain countries and do not want to disappoint their customers so they can keep getting repeat orders.
But if you're gonna go through all the trouble of getting onto the dark net, there's far more potent substances you can purchase for hopefully a more comfortable and peaceful CTB (stuff like barbiturates and certain opioids). While I haven't tried searching for SN on the various dark net markets myself, because it's still not too difficult to order from the clear net, I'd be surprised if any of them were selling SN, especially since almost all (if not all) vendors will only being selling stuff like drugs to make people high/feel better not for the purposes of CTBing. Most if not all dark net markets have even banned the selling or lacing of Fentanyl in products due to how many accidental deaths they were getting with people using Fentanyl.