Usually you do not have to pay custom tax for a $500 package shipping from China to USA based on its customs policy.

Once you order the goods from China to USA, if the customs suspect your package that exceed $500 value, you will be asking to provide the goods invoice as well as the transaction records, if you could provide formal documents then you will not likely to pay tax, else the customs may compare goods with the local market value if it is exceeded the tax value that USA customs regulated, you may need to pay tax.