Is it possible to payback my HSA? I used my HSA to pay a medical bill in 2019. I now have the money to pay that bill, and would like to keep my HSA buffed up
No, you can not "payback" your HSA unless you are correcting a mistaken distribution.
You can however contribute for prior years until the tax deadline, which would essentially be the same as what you want to do. So if you want to add money for 2019, you have until April 15, 2020. You will have to specifically designate that the contribution is for 2019.
Keep in mind that you must still be below the contribution limits for 2019 - $3,500 for individuals and $7,000 for families. If you have already contributed the max for 2019, then there's nothing you can do.
Your HSA has an annual contribution cap. You cannot retroactively make contributions for the previous year after April 15th of the current year.
This amount changes from year to year. More information can be found in the IRS publication 969.