Too dark print with Costco. How to lighten up?

What are the correct post processing steps for preparing images for online submission to Costco using Lightroom 4?

I am working with NEFs in the SRGB colour space in the Develop module. I use the export feature to create JPEGs in LR4, submit them online, and check the box for Costco to not adjust them. However, they came back darker and the colour is not as true as what shows on the monitor.

Afterward I soft proofed some of those pics using the Costco glossy profile for my store and they look even darker than the original ones I sent. Am I doing something wrong? If I send those proofed JPEGs (as dark as they are) will they come back any brighter?

My monitor is profiled using the Spyder 4 Pro tool. I reprofile it every 2 weeks in medium ambient light.

Thanks,
Brent