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I purchased from T-Mobile.com without issue. In fact, I recommend that route because last time I checked, if you buy it through Walmart you're contractually obligated to use it with a phone you buy from Walmart (though I have no idea how well this is enforced).
As for going beyond the 100 minutes, they'll charge you per-minute past that as long as you have the money in your prepaid account to cover it. Last I heard that rate was 10¢ a minute (domestic only, of course), but naturally being off-contract that may have changed.