Thanks for your interest in our laser cutting. You're probably OK just going with 3mm acrylic. It's cheaper to cut than ABS, looks nicer, and comes in a variety of transparent or close-to-opaque colors. Just be sure not to screw anything down too tightly (maybe some washers would help) since it is more brittle than ABS (which is softer and has some give).
In case it matters to your design, note that the acrylic thickness can vary by + or - 10% even within the same sheet.
For the reference of future readers, here are links to the material descriptions that you probably read already:
You can see pictures of some of the 3mm acrylic we stock by scrolling down a bit at:
Hope this helps; good luck!