WOW, 5000 reward points, is it mistake, I thought it should be 500 points. By the way, I notice Drop charge tax before discount, which is not right. They are overcharging tax, the tax should be based on 'after discount'.
iucoenhttps://drop.com/help/rewardsprogram, the official statement: Once you have your Drop Credits, they’ll automatically be applied as a discount against your next purchase. By the way, Drop used to charge tax after discount. However, since August they suddenly charge tax before discount.
ciberCheck for any change in your states sales tax laws, that is usually where that type of change comes from. If Drop has no physical presence in your state they may bypass sales tax all together.