The add-ons are where the total lands. The 90-day e-visa through the desk is 2,400,000 VND single entry or 3,100,000 VND multiple entry, filed the moment your exit is stamped and chased until approval. The Cambodia visa add-on is 1,100,000 VND, about 43 USD, covering the arrangement and the queueing; the government fee at the Bavet counter is still paid in clean USD cash. Guided assistance, if you want someone physically walking you through both borders, is 700,000 VND more.
Booked together, transport plus both visas is 5,300,000 VND, and the full bundle with assistance is 6,000,000 VND. Against the DIY day with a new e-visa, roughly 3,400,000 VND all in, the gap is about 1,900,000 VND, roughly 63 USD, for a day where nothing is your problem. Pay on arrival, cancel free up to 24 hours ahead, and the price you see on the cost page matches the booking form. That is the version of cheap that survives contact with the border.