For poster printing, Office Depot in Issaquah seemed the best for the following reasons:

• Price for 18"x24" on foam core is $20
• Accepts several file formats
• From order to pickup is around 2 hours

Note: 18"x24" in landscape mode is a perfect fit for the sandwich boards.

Steps to order a print in Issaquah:

• Select the Issaquah store as "my store" (Store #975)
• Select "Copy and Print" from top menu
• Select "Posters"
• Select "18 x 24"
• Select "Fit to size" Note: image might appear cropped as if in portrait mode, but a human at Office Depot adjusts the file by eye
• Select Semi-gloss paper at $15.99
• Click on "Foamcore mounting" under "Finishing Options", which adds $9
• Upload file
• Repeat as needed for additional files

Note: try to upload images that are rendered at 72dpi for printing.