Sounds like a good plan!
As per the flakes...
I've actually got 2 different brands...why? I dont know, they both look pretty much the same -- One made by wardley, the other my tetrafin. I can't tell you which brand is best, as my fish love them both and again they seem pretty much the same.
Most of aquarium "kits" that come with food samples include a small sample of goldfish flakes, so it's likely the stuff you have is the same stuff we're talking about.
As for the seaweed, I can't really help ya there. Never tried it.
If you've got romaine lettuce (not iceberg), cucumber, or zucchini you can try feeding those to your goldies. Some will eat peas as well--just take the skin off the peas first, and squish them between your fingers. Any of those would be cheaper than $4 for seaweed
Just make sure you remove the excess food that is not eaten after a few hours, otherwise it'll start to rot and mess up the tank.