This week brought some welcome news for Amazon Prime shoppers in greater Rockville and Potomac: The company just launched same-day delivery service in the greater D.C. area, including area codes 20850, 20854, 20817 and many others nearby. You can now order by noon for guaranteed delivery by 9 p.m., or by midnight for delivery the next day — as long as you spend at least $35 and choose items marked with the Prime Free One-Day logo. The list of qualified items is not exactly comprehensive, but we spied plenty of things (batteries, printer cartridges, kids’ birthday presents, etc.) that could easily spare us from a last-minute shopping trip. Same-day shipping is free for Prime members, but it’s also open to other Amazon customers for a $9.98 delivery fee. And here’s some more good news: Amazon says it will soon start offering free shipping for everyone on inexpensive items that weigh less than eight ounces.