best way to mail an extension

Why not efile it?

What is a registered-mail receipt going to do, other than show you mailed something (well, maybe an empty envelope) to IRS, where often there is no one to sign for it anyway?  It could be a pair of used underwear.  

The cheapest way to get a tracking number is with a First Class package -- under $3.  If you don't print your own labels, the postal clerk may give you a hard time about it being too "flat."  Maybe add a pair of new underwear.  Next is Priority Mail, about $7. Certified is going to be $10 or $12 if you want the green postcard back, which you really don't need if you have a tracking number.  Registered, which the Post Office discourages because it's overkill and involves a lot of extra work, runs $25 or $30 last time I checked.  Not that I check very often. If it's that valuable, use FedEx or UPS, although they have been having problems lately also. 

If you expect a balance due, include a check even if a small amount. Mail it early enough with a 55-cent stamp (or 51 cents if you print your own) and make sure the check is cashed before Aprli 15.