While you do have the option to invest every cent of what you contribute into your health savings account, starship heavily discourages this. Any card transactions or reimbursements pull from the Spending Account balance. If you invest your total balance (moving it out of the Spending Account), there will be no funds available for card purchases or to readily reimburse yourself. You would have to first move funds from your Investment Account back to your Spending Account. It can take 3-5 business days before the funds become available for you to spend.'
If you choose to auto-invest your entire Spending Account balance you would want to set your auto-invest threshold at $0.