There can also be many other variables. Companies that advertise that you can achieve a specific response using their lists, are best kept away from. There is no time limit on the use of the names, provided that they are used only once. However, the older the names become, the less responsive they become. Only a detailed analysis of mailings of your offer using both bulk and first class will tell you which method is ultimately most profitable for you.
Bulk mail is less expensive, but first class mail has an aura of legitamacy.
First class mail is more likely to be opened by the person who receives it. Minimum orders are normally 5000 or more names. Where a list is smaller then 5000 names, the minimum order is the whole list. The real purpose of these ads is to generate cheap names that will be rented to you. The real problem is that the people who answer these ads are not the serious people you need.