How old is your bunny? You need to spay her before introducing another rabbit, to avoid fights but also babies (you really can't trust sexing done by petshops). You need to be aware that female rabbits need to be spayed around 8-10 months for health reasons, so even if you end up with 2 females you'll need to pay for two spays (do you have enough money for that? assuming your in the US, it might cost around 400 $).
Also, will you be able to keep the rabbits separated if they end up not getting along? Two females can be done, but it isn't an ideal pairing.
How much time outside of her cage does your rabbit have? My pair live in a cage which is roughly the same size than yours but my rabbits have 16 hours outside of their cage everyday (they are only confined when I'm sleeping). I'm not really partial to keeping a pair in a cage for most of the day because the rabbits can't isolate themselves if they want to (it really depends on the rabbits' personnality - Aki likes to have her "alone time" during the day) and until the pairing is really stable it bothers me to keep a pair in a confined place because if any fighting occurs, the weak party won't be able to flee...
So I guess it all depends on your rabbit, your general situation and how much you are willing to "sacrifice" (money and potential time) for the rabbit.
Nevertheless, I feel like I have to point out that playing on the feelings of people is the way petshops make money - it's not the best place to get a bunny as it encourages a pretty nasty industry (I don't know where you live, but in France most petshop's money go straight into the eastern europe countries and partially funds 'business' like forced prostitution, I'm not even talking about the conditions in which the animals used for reproductive purpose are kept or the fact that most of the animals brought to petshops don't survive the trip). If you want a second bunny and wish to save one, I would rather take a nice neutered male bunny from a rescue.
If you decide to take the petshop bunny, don't forget to quarantine - things like mange run rampant in petshops. Bring her to the vet quickly to assess her general state and make sure she is indeed female. Also, you'll have to introduce the vegetables very slowly as most petshop rabbits don't have access to greens (their parents either) so their stomaches have trouble digesting natural food (the multiplication of petshop bunnies has caused this legend that young rabbits can't digest vegetables, which is of course not true if the rabbits and their mother were fed as they should be).