Learn Arabic (Modern Standard): ألغيت البطاقة لئلا يقتطعوا المبلغ تلقائيا. - I canceled the card so that they do not deduct the amount automatically.

It is a contraction of three words: the prefix لـ (so that), the particle أن (that), and the negative particle لا (not). Together, they mean so that not or lest.

Why is the verb written as يقتطعوا instead of يقتطعون?

Because of the أن hidden inside لئلا. The particle أن forces the following verb into the subjunctive mood (mansub). For present tense plural verbs, this means dropping the final ن and replacing it with a silent alif.

I know the root ق ط ع means to cut. How does the verb form used here change that meaning?

The verb يقتطعوا is a Form VIII verb, which you can spot by the inserted ت after the first root letter (ق). While Form I means to cut, Form VIII often means to take a part of something for oneself, which translates here as to deduct.

Why does تلقائيا end with an alif and tanwin?

The base word تلقائي is an adjective meaning automatic. By putting it in the accusative case (mansub) with the ـًا ending, it functions as an adverb, changing the meaning to automatically.