1. No one time. You have 50 BTC invested.
2. You take 1 BTC of your invested stack and buy whatever product you want
3. The you rebuy 1 BTC with fiat
So you only spend 1 times the fiat cost of the item you bought (although probably less due to all the benefits of Bitcoin)
Hmmm I'm sorry but how does this not entail spending 2x the amount of money, fiat or BTC.
Depends upon definition... Of course, you have spent twice;however, one of those spending is a form of internal transfer from yourself and to yourself. The other is a transfer from yourself to another entity... Accordingly, there is only 1x spending from yourself to another entity.
but you have to pay the fee even if you are "spending from yourself to yourself".