Steve,
The Haynes instructions are almost identical to the ones from the Volvo Service Manual (TP 4305201) for the AW 50-42 tranny, here are the instructions for fluid change: (for automatic tranny)

1. Remove protective cover under engine and remove drain plug...drain oil from the transmission.
2. Reinstall oil plug with a new seal, torque to 40Nm (18ft/lbs). Reinstall splash guard.
3. Remove Battery and battery tray
4. Detach return hose on the control system cover. Insert a plug into the connection on the cover. Install a transparent plastic hose on the oil return hose connection. Place a container beneath the hose to catch the oil. Temporarily replace battery tray and battery.
5. Set the parking brake and set the gear shift selector to position P. Add approximately 2 liters of oil. Start engine and allow it to idle. Turn the engine off when air bubbles start to become visible in the hose. Add approximately 2 liters of oil. Start the engine again and allow it to idle. Turn the engine off when air bubbles start to become visible in the hose. remove battery and battery tray. Remove hose from connection. Refit oil return hose to transmission cover, replace battery tray and battery.
6. Add approximately 2 liters of oil. start engine and let it idle, move the gear selector through all positions, stopping in each position for 4-5 seconds.
7. move the gear selector to position P with the engine running. Wait 2 minutes, then check oil level. Top up if necessary. The transmission holds 7.6 liters.

Steve, the quote you got for $90 is very high, the dealer changed mine last year for $60cdn! ($45 U.S approx). Some jobs are just to much of a headache...let the dealer do it. --
Tony G...'94 850GLT, '82 240DL (mint)