Since this formula's own guarantee is calculated from your shipping date, your order confirmation email is worth keeping accessible from day one.
Your order number, the shipping date, the exact package and price you paid, and any correspondence with customer support — all worth keeping in one place, not just your inbox.
Since results are described as building across a couple of months, having your shipping date clearly documented protects you across the full 60-day guarantee window this formula's own materials describe.