Servo extensions are much more expensive than cutting and lengthening the wire by soldering in additional wire.
If you don't know how to solder it is probably best to just pay the money and buy the extensions.
I have never had an issue with a servo extension plug oxidizing or corroding, I would think that if the environment created this issue it would not be localized to the mated connectors.
Here are examples of the three most common solder connections for inline wire splices.
If you search the internet on how to solder two wires together you will find many many examples that expouse twisting the wires together to create a mechanical connection and then applying solder to hold the joint in place, there are alot of reasons I don't like this method of making an inline wire splice. But this method is so prevalent and so many people believe this to be "the" way to do it that I am not going to debate it.
It will work fine but think about this, it creates a right angle stress point that will predictably fail the quickest in a pull test and more importantly to the RC application it creates a big bulky joint that is prone to snag when pulled through a model airplane. On the plus side it is probably the easiest connection to make since twisting the wires together creates a stable place to apply solder.
The wires soldered together using the hooked ends is technically the 'correct' way to make an inline splice, but for this to be effective correct tinning and bend radius has to be taken into account and again for RC applications it does not offer any significant advantage over the parallel wire splice.
The other example is the parallel connection. While there is no mechanical advantage of twisting or hooking the wires together, it creates the least bulky joint which allows the wire to most easily be pulled through a model airplane. It also creates the least stress angle that otherwise defines the break point of the other joint types in a pull test.

Which ever method you choose if you make the solder look like the solder in the pictures then you have applied the solder in the correct quantity and at the correct heat setting.
In the series of photos four wires (AWG 22) were soldered together using the three different solder joints to make one long wire spliced together with inline splices.
Pic #1 shows the three solder connections immediately after they were soldered together.
Pull pressure was applied to the two end wires stressing all three solder joints at the same time, the pressure was increased until the first joint failed and a photo was taken of the failed joint and the remaining two.
Pic #2 shows all three joints after enough pull pressure was applied to cause the first failure.
Note that the insulation is now pulled farther away from all the joints due to the copper stretching, so even the first joint that failed is a significantly strong joint.
Pull pressure was again applied until the next connection failed and a photo was taken.
Pic #3 shows two failed joints and the last remaining solder connection.
The copper is stretched even more which makes the insulation be even farther away from the soldered connection
Pull pressure was agin applied until the wire failed and a picture was taken.
Pic #4 shows all three failures, note that the wire broke before the last solder connection failed.