Depends on the coin value and the amount I charged for shipping based on that value.
If it's a common, circulated low value single coin, it's getting stapled into a 2X2, taped to a 3X5 index card and mailed via US first class.
If it's a low value type or date set, they get bagged into a small ziplock, taped to a cardboard insert, and mailed in a small bubble mailer.
Higher value items get packaged better and always get delivery confirmation. Always, always, always use delivery confirmation on anything you aren't going to like refunding if it doesn't show up (and not just coins, either. It's amazing how many times people have claimed non-delivery only to shut up once the completed DC was brought up.). Delivery confirmation is your proof of shipping and once the PO says it's delivered, you've done your part. Paypal considers a completed delivery confirmation as proof of due diligence on the sellers part and will take the shippers side in disputes concerning non-delivery claims.
Plus it's CHEAP to use.
Lost packages are RARE in the USPS system.
Shipping overseas is always a crap shoot though, especially to Eastern Europe and some places in Asia.