This has been a point of discussion a few times, and postal clerks give inconsistent answers. The safest answer is the package rate, but some people are more aggressive about saving money and use regular envelopes and ordinary first class mail (which would make it around 45 cents or 65 cents, depending on the risks you take).

If you use the package rate ($1.95 at post office; $1.64 on-line with free delivery confirmation), keep in mind that the package has to be at least 3/4" thick for delivery confirmation purposes or they can tack on postage due to bring it up to the priority mail rate (which is around $5) - so put extra padding in that envelope to make it 3/4 inch.