You can get arrested for theft and/or fraud. This is not some new way to steal items; changing price tags started approximately the day after price tags were invented. Some state laws handle it explicitly, some implicitly; for instance, Maryland defines "deception" in its theft statute to include "(vi) remove or alter a label or price tag;" theft is committed if, among other possibilities,
(b) Unauthorized control over property - By deception.- A person may not obtain control over property by willfully or knowingly using deception, if the person:
(1) intends to deprive the owner of the property;
In other states, caselaw says that switching price tags is deception, and doing it for gain is fraud. See this California case in which switching price tags and buying the item is completed generic "theft by false pretenses" if the cashier didn't know you had switched the tags and relied on the new ones (in that case, the cashier knew so it was just attempted theft); see also this Nebraska case. Notably, the deception there is not tied to a statute saying "switching physical price tags is deception;" rather, it's deception because it involves knowingly making a false representation of a material fact (i.e. the true price of the goods) with intent to fool the store into thinking the real price is the lower one.
As a general rule, many criminal laws handle new technology by looking at how you're using it. If what you're doing would be flagrantly illegal if not done on a computer, it will likely be illegal if you use a computer.
Because you used the Internet, you might theoretically face further charges. If this is considered to be deception, you could in theory be on the hook for wire fraud. This is a federal felony offense. While small-scale offenses would more likely be prosecuted at the state level (and if you are federally prosecuted for one small fraud you'll probably face 0-6 months in jail instead of the 20-year maximum sentence for wire fraud), it is in fact a federal crime to commit wire fraud.