(LA Times) — Vincent and Liza Concepcion bought cellphone service from AT&T Mobility in Southern California because the offer included a free Nokia phone. They objected, however, when they were charged $30.32 in sales tax. This hardly sounds like a federal case, let alone a Supreme Court landmark, but it could decide the legal rights of millions of American consumers. At issue is whether they can join with millions of others in a class-action suit against a company if they believe they were cheated out of a small amount of money. Many companies are determined to shield themselves from suits that could turn a $30 claim such as the Concepcions’ into a $30-million court case. The fine print that comes with their products often includes a clause that says the purchaser agrees to take any complaints to an arbitrator and may not join a class-action suit.