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are the paint bundles actually a savings?

if not,

then how does a paint bundle sell out?

Do you mean this bundle?

Depending if the picture is accurate (i.e. if the Shade is in the old-style pot and the same size as the rest of the paints), then it's less cost-effective than buying them separately.
It's likely just a photoshop lineup though, if it's a full-size Shade pot then it's exactly the same as buying them individually.

You'll often find that bundles on the webstore don't actually save you money. They just save you mouse clicks (and act as suggestions of what to group together).

No idea how that sells out though. Drive up demand by making it look limited probably.