When you select registered, it carries its own insurance. When you select registered, the same window as for insured pops up to select the amount of insurance you want on your registered package except the insurance rate is considerably cheaper.
The Registered fee includes insurance to the declared value. The Priority insured is the flat rate fee and when the fee for insurance is added to that it comes to the $30+ figure. If you go with first class package rate and insurance, the insurance fee is the same but the postage cost for a first class package is higher than the Priority flat rate. ($6.75 to $7.85 depending on what zone it is going to for first class or $5.80 for flat rate priority.) So Registered is slowest and cheapest but also the safest, Priority and First class are faster (The same or almost the same speed) with Priority cheaper and first class the most expensive option.
Registered mail actually has to fall into either the priority or first class category as well. "Registered" is an add on to either of those two classes of mail. And first class is almost always cheaper than priority.
True, but it is usually done with the priority. And first class package rate is HIGHER than priority flat rate. The figures I gave above was for first class package of less than one pound. With Registered if it is over 13 oz it HAS to go at the Priority rate.
