I use my HSA as a retirement vehicle. I did lots of research on this, and ended up at
Saturna. It was the cheapest option I could find that offered index funds, and didn't require any money sitting in cash before I could invest, which has it's own opportunity cost which is it's own fee.
This thread was particularly useful. Here's the TL:DR. Make your investment once a year, and all you'll have to pay is the "cost per trade" (once a year), and any dividend reinvestment fees (probably 4 times a year).
Vanguard Mutual Funds
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Cost per trade: $24.95
Admiral Shares: Yes (*without* meeting the minimum 10k requirement).
Dividend re-invest: no charge
Inactivity fee (no trade within calendar year): $12.50
Fidelity Mutual Funds
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Cost per trade: $14.95
Admiral Shares: Yes* (but *will* need to meet 10k requirement). The new website will give you an error right away if the order doesn't meet the fund's minimum.
Dividend re-invest: no charge
Inactivity fee (no trade within calendar year): $12.50
ETF (Vanguard & Fidelity)
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Cost per trade: $14.95
Dividend re-invest: $1
Inactivity fee (no trade within calendar year): $25