Expedia does not "issue" airline tickets nor "rent" hotel rooms - it is an online travel agency - the airlines set the fare rules, and each airline fare rules for each fare class are different - there are no standards, although generally speaking, the "legacy" airlines have far more restricted rules (AA, UA, NW, DL, US, etc) versus the discount airlines have more lenient rules (Southwest, Spirit, Jetblue, although AirTran has very restrict rules too, etc etc)
When you purchase tickets, make sure you know the FARE RULES for the class of fare you are purchasing - there are gazillion of fare classes and sometimes there are only $20-30 difference between the rock-bottomed and the ones that can be changed with a fee (or upgradable, or earn status qualifying points, whatever). Sometimes a ticket can have several fare classes if there are multi-segments.
Whether you book your ticket on Expedia, Travelocity, Airlines own sites, make no difference, the corresponding FARE RULES govern !
Expedia actually is fairly good in honoring pricing errors if made by them.
Last November, I took advantage of a pricing error to book Hilton Osaka for USD3.00 a night for Aug/Sept (because during currency conversion, the computer dropped of 2 zeros from JPY). Expedia offered to pay the normal rate of USD300 out of their own MasterCard if I wanted to go. I did not really want to go, and at the end, Expedia offered me $200 coupon for taking a cancellation, plus another $200 to compensate the fees involved in changing award ticket booking.