As a buyer I can state that there were indeed minimum prices in effect for a long time, I've been here since 2001 and a $5.00 minimum was around for most of that time.  That said it is possible that in the past when special sales were run that the minimum was lifted (I actually seem to recall that being the case several years ago).  However it doesn't make sense to do that with a minimum of 3.50 because the cost to process the payments would make that too costly, regardless if some choose to use a money order or cheque the system would not recognize a difference and then lift the minimum.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo