The pet store should have treated your guinea pig with Ivomec, which kills both internal and external parasites, such as worms, lice, and mites. Go back to the pet store and demand that they contact a veterinarian to administer a dose of pour-on Ivomec. This is the recommended parasite medication for most animals, including guinea pigs. Do not give your guinea pig any dewormer by mouth or by injection. If the pet store refuses, then you have two options.
1. There are shampoos which are specific to get rid of lice, but it requires several treatments and guinea pigs are not particularly fond of baths. You should never get water in the pig's ears or eyes, so you it's very difficult to get rid of the lice on the head. When a guinea pig is wet, it gets cold, so toweling dry is not enough. You should use a hairdryer on the lowest heat setting and gently dry the pig (make sure to get his belly too!).
2. Take your guinea pig to a veterinarian. A veterinarian can take a sample by lightly scraping the skin and looking under a microscope to determine if your guinea pig has lice or mites. Mange mites are deadly, so it's very important to know if there are mites.
The veterinarian will treat your guinea pig with pour-on Ivomec. Because your pig had worms, you will need to repeat the treatment again at 3 weeks and 6 weeks.
After you see the veterinarian, take the bill to the pet store and ask the manager for reimbursement. If the manager refuses, then file with your local small claims court because you will win the case hands down. Filling out the form is easy and you will be reimbursed your court costs if you win. Most likely, if you file in the small claims court, the manager of the pet store will want to settle it instead of risking a loss in court because that looks very bad for business.
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If you prefer not to go to small claims court, then you can send a copy of the vet bill with a letter to the Attorney General's Office of the state where you live. They will investigate the complaint and most likely, order the pet shop to reimburse you for the vet bill. The Attorney General's Office also has the authority to refer your complaint to the prosecutor for further action regarding the condition of the animals for sale at the pet shop.
Because your pig has such serious problems - all of which can be cured - you need to see a veterinarian as soon as possible. Do not delay this for more than a week or the problem will just get worse. While you're waiting to see the vet, thoroughly clean the guinea pig's cage and anyplace in your home where the pig roams around. An inexpensive, effective cleaner is Ajax liquid floor soap mixed with water in a spray bottle. Spray the areas you want to clean and wipe with a damp cloth. Throw out any used shaving in the cage and throw out any used hay. If you have towels or a little bed for the guinea pig, thoroughly wash the items. You will have to keep the guinea pig cage very clean until the end of the Ivomec treatments because mites and lice will hide in the pig's bedding and hay. Do NOT use a commercial pesticide because the residue is toxic to your guinea pig!
Finally, if you happen to know someone who raises sheep or goats, they might have some pour-on Ivomec at their farm. Your guinea pig only needs a tiny bit, and you can look up the dosage on http://www.guinealynx.com That would be the only "cheap" way for you to treat your guinea pig for parasites without seeing a veterinarian. However, it would be wise to have your guinea pig thoroughly examined by a vet so you don't have any more "surprise" health problems.