Hmmm...Starbucks does this by charging me a lump sum in one transaction and then allowing me to spend it down over time in smaller chunks.

I doubt this is viable in general though, due to the cited support issues around reversibility.

Oh, that is even worse! Only big corporations can afford to do this, as this borders on banking services and you need good legal grounding for this type of business.

I should probably clarify that by "impossible" I really meant "does not make business/economic sense". You can do microtransactions with individuals, but if you're spending €2 (rough estimate) per transaction for invoicing, accounting, VAT processing, etc, it no longer makes any sense. Thus "impossible".