The IRS sets an interest rate each quarter that is applicable to underpayments by individuals. The rate for the third and fourth quarters of 2020 is 3%. You can see all interest rates listed by the National Association of Tax Professionals at https://bit.ly/3bVcCmw.
For 2007, a high deductible health plan is a health plan with an annual deductible that is not less than $1,100 for self-only coverage or $2,200 for family coverage, and with annual out-of-pocket expenses that do not exceed $5,600 or $11,200, respectively.