VAT rates come straight from your Shopify settings
Feeds no longer guess the VAT rate: the default rate is read from your Shopify tax settings, and per-product overrides flow into the channels that price with VAT, Alza included.
A feed used to assume one flat VAT rate, typed in by hand. If the shop's real rate differed, or differed per product, prices including VAT came out wrong, and channels that verify prices against your landing page noticed before you did.
The default rate is now read from your Shopify tax settings, and per-product tax overrides count when Feedflock decides what extra data a feed needs. The price in the feed is the price Shopify would charge, not an approximation of it.
Channels that want exact per-item VAT get it: Alza's feed now carries VAT per product, shipped together with its rebuilt availability feed, so the stock and the tax on Alza both match your store, not a default.