For example, if you paid early and declared $120 in gross sales for the March 31st period but you realized at the end of the period sales were actually $140.
That means you paid tax on only 85.71% for that period which is below the 90% threshold.
That means you must pay interest on the tax for the 4.29%, (90%- 85.71%), difference. 90% of actual sales $140 is $126. Therefore, the tax on $6, ($126-$120) will face the 15% penalty.