Yes, you can set this up as a redirect. Your custom domain sends clients to your storefront on the Altro domain, so you keep a branded entry point while the storefront and checkout run on Altro.
Checkout happens on the Altro domain for many reasons, including that processing medication payments requires LegitScript certification and compliant payment infrastructure, which Altro maintains.
Alternatively, if you prefer a section of your website to link to your Altro portal, click here to learn about this.