Actually, it is based on shipment minus returns. AB is certified Gold, but it takes a while for Certifications to be made public. From RIAA.com:
We are often asked why we don’t just use sales figures from SoundScan. SoundScan measures over-the-counter sales at music retail locations, while the RIAA®'s certification levels are based on unit shipments (minus returns) from manufacturers to a wide range of accounts, including non-retail record clubs, mail order houses, specialty stores, units shipped for Internet fulfillment or direct marketing sales, such as TV-advertised albums. The other difference is that SoundScan's archive is only a few years old, while the RIAA® has tracked artists' sales levels for more than 40 years.