The search results for Vietnam e-visa are full of sites designed to look official. They take the same information, submit the same application and charge you more for it.
If you would rather somebody handled it, that is a fair thing to want, but pay an agency knowingly rather than by accident. One question sorts it: what is the total I will pay today, including every visa fee?
For extensions once you are here, Lynn Visa in Da Nang came to 3,900,000 VND all in when it was last checked.