As an account manager you can switch your active cart to a member's account, but only on the Subscription and My Meals pages — no other ordering module supports this. If the member is your own account member, the switch works; if their cart already has items from any module other than Subscription/My Meals, the switch is blocked until that cart is cleared. Separately, whether a purchase is billed to you depends on the "You pay this member subscription" setting on that member's profile: if it's on and you haven't also checked "Member is allowed to checkout in «Weekly Subscription»" (or My Meals) for them, the member is blocked from buying a subscription or picking meals under their own login — they'll need you to do it for them, or you'll need to grant them that permission.