To undercharge is to ask someone for less money than you should have. The shop, restaurant or worker makes a mistake — or sometimes decides to be kind — and the customer pays too little. You can undercharge a person ('she undercharged me'), or undercharge for a thing ('they undercharged for the repair'), and it is often used in the passive: 'I think I was undercharged.' Notice that the mistake belongs to the seller, not the buyer: the seller undercharges, the buyer is undercharged. The word is neutral in tone, not an accusation — it usually describes an honest slip at the till, and English speakers often use it to admit their own error politely: 'Sorry, I undercharged you yesterday.' It also has a wider business use, where a company undercharges for its work meaning it sets its prices too low to make a profit — this is about pricing policy, not a one-off mistake. The opposite is 'overcharge', which is much more common, partly because people complain more loudly about paying too much. There is a separate, rarer technical use about putting too little electricity into a battery or too little explosive into a hole, but you will meet those only in specialist writing. As a noun, 'an undercharge' means the amount that was missed.

少收（錢）、算便宜了、收費不足。台灣店家口語常說「算錯了、少算你的」，這個字就是那個動作；相對的 overcharge 是「多收、算貴了」。