This site states this directly beneath its research-sources section: secure, encrypted checkout. One-time purchase — no autoship, no hidden fees, ever.
Pairing a checkout-security claim directly with the no-autoship, no-hidden-fees claim is worth reading as this site addressing two separate buyer concerns — payment security and billing structure — in the same statement.
Worth confirming this yourself at the actual checkout page before entering payment details, rather than relying on the homepage claim alone, particularly given how many similarly named domains exist in this category.