Save your money.
The ONLY test that will accurately tell you the sex of your child 100% of the time is an invasive one, like an amnio, where they actually test the dna floating around in your amniotic fluid.
I seriously doubt you want to do one of them just for convenience sake.
Instead, de-emphasize the sex of the baby to your daughter. Spend a lot of time talking about how she is going to be such a great big sister, what big sisters do, etc, and gently add in that she'd be happy if she had a brother *or* a sister, right? That kind of thing.
Not sure how much 'work' you've had to do in planning for a second girl, since you already have one. I'm sure you are re-using all the clothes, etc. Any decisions about decor in the baby's room are not going to be important to the little one for at least a year or two, so you have plenty of time to make any changes if you do end up having a boy.
Since ultrasounds are not 100% sure of gender (they are mainly looking at the health/development of the baby, gender is a perk), not sure why you would need to 'keep your reciepts' either. 80% isn't really that high a percentage of a guess, and there is no 'guarantee' that will get you your money back for guessing the gender wrong.
In the meantime, you have two months to work on a gender-neutral message with your daughter. I would recommend that any close friends and family you've told that the baby is a girl, that you communicate that it is only an 80% guess, so there is a decent chance you could still be having a boy. Anyone who is planning on buying a very pink/girly gift might want to wait until after the baby is born. Talk about boy names with your DH a bit, to get an idea of how you each feel about boy names (don't want to have to think of some under pressure at the hospital right after the birth.)
Better to be prepared and then pleasantly surprised if it is a girl!
Good luck!