Would love to pick up a few of these, but no luck so far.
I have had this same problem with both batches that just went on sale and the batches last week.
I screenshot the "proceed to payment" page where that option is "greyed out" and sent it to Pangolin and they responded to tell me that the cause of this is just that the item is out of stock.
The miner's were just selling out very quickly...Within seconds of release.
Pangolin's website says more M3's are going on sale again in about 24 hours so I wish you luck getting some machines.
Despite the noise and power consumption I think these are good value when compared to the current prices of other miner's.