Hate to break it to you, but both "drive-through" and "drive-thru" are considered correct.

Wikipedia refers to "A drive-through or drive-thru (a sensational spelling of the word through)..."

As explained on Erin Wright Writing, "According to our primary American-English dictionaries (Merriam-Webster’s Collegiate Dictionary, Webster’s New World College Dictionary, and The American Heritage Dictionary of the English Language), drive-through is the preferred spelling, while drive-thru is an accepted informal variant. However, the use of drive-thru has become so common that using drive-through now feels incorrect."

Also keep in mind this is Xitter, where abbreviations are are a normalized (often necessary) part of the lingo.