Is this a complete sentence?

[still learning]

Doing so would reduce your registration fee from $2200.00 to $1800.00.

Thanks

[RonBee]

Yes, indeed! That has everything it needs to make it a complete sentence.

Welcome to the forum. :D

8)

[MikeNewYork]

Doing so would reduce your registration fee from $2200.00 to $1800.00.

Thanks

I assume that you are confused about the subject of the sentence. In this sentence, the subject is "doing" (with so as a modifier). This is a gerund, a verb form (verbal) that acts as a noun. Gerunds are quite common in English. They allow one to talk about an action as a thing.

Swimming is very enjoyable.
I love reading.
Walking is good exercise.

Gerunds always end in -ing. Unfortunately present participles of verbs also end in -ing. This results in confusion.