If you placed an order with Amazon.com this week, you might have noticed an unpleasant surprise: On Wednesday the company started charging Maryland residents 6 percent sales tax. Now that Amazon is building a distribution center in Baltimore — which gives it a physical presence in this state for the first time — it can no longer waive taxes for online sales in Maryland. The change provides a more even playing field for competing discounters, like Walmart and Target, that have brick-and-mortar stores here.