Treatment-heavy means buffered: keep vitamin C to the morning and retinol to the evening, and don't stack BHA with retinol. Mixing them irritates skin. The routine assumes a baseline of healthy barrier function; if you're flaring or sensitized, run the Sensitive Reset for 6 weeks before introducing actives. Timeless Vitamin C + E + Ferulic uses the same three antioxidants as SkinCeuticals C E Ferulic at a fraction of the price (Skinceuticals isn't sold on Amazon). COSRX The Retinol 0.3 Cream is the cheaper retinol alternative.