Yes, you can invest the funds in your HSA once the balance in your account reaches $1,000. Any amount over the $1,000 can be invested. The same types of investments permitted for IRAs are allowed for HSAs, including stocks, bonds, mutual funds and certificates of deposit. Visit the PayFlex website or call them for more information about investment opportunities.
Even if you do not invest the funds, the money will earn interest at the current rate. The interest rate as of 8/31/17 is 0.05% and increases as your deposit account (the non-invested portion of your HSA) balance goes up, but is subject to change quarterly, as determined by the HSA administrator.