We take probiotic supplements in pill form. Just pop the pill with some water and you are set. They are a bit pricey, but worth it in the long run. We buy from vitacost.com - they have the best prices and very high quality supplements. Most of the stuff on the shelf at Walmart has very little effectiveness. They have a flat 4.99 shipping fee, but that is offset by the price of their products.
Also, my in-laws were feeding their dog this expensive, vet prescribed dog food for her intestinal issues. She was abused by a previous owner, had terrible worms and flees. I started giving her a pill a day when we watched her and her “pooping issues” were resolved. She is off the overpriced prescription dog food and the vet is pissed!
If the vet is upset that the pet is healthy, your in-laws need to find a new vet. The DVM that I use for my vast collection of critters wants them healthy. One of my dogs improved dramatically when I started cooking her food and the doc was tickled pink. If I ask him a direct question about getting meds and treatments from a substantially cheaper source, he helps every time.