"When spot colors are used with transparency..."

This resolved a complex issue for me. Thank you. 

I had the same error, and I was able to solve it thanks to your publication, so thank you!

as you say time crunch...

usually stuff gets printed as 'process'

where the inks are Cyan Magenta Yellow and Black - 4 inks,

the 'process' is the images and text being made up using a mix of those inks.

Spot colour (Pantone Colours etc) are used to introduce a special specific ink colour,

so they literally stick a pot of orange for example in the system, so that your print is made of process + 1x spot ink,

or many more if you added lots of spot colours.

for your jobs it sounds like the best plan is just to get illustrator to convert those Spot colours (as te_co) explains, in your design to the nearest matching colour that can be printed using a mix of CMYK inks.

as far as the print job goes, additional spot colours cost more as there will be an extra plate, extra ink, another stage in the process, and often a different machine to run it on so it all adds on to the cost. so you generally only use spots for colours that can't be represented as well in CMYK, or to match more accurately and get better Solid looking colours especially noticeable in light and bright colours.

A pantone book is a great help for choosing colours and finding ones that work well when you have to use CMYK only to print.

you are likely not printing with spot colors if you don't know what they are.

in the swatches panel you can select all the spot colors, select the 'swatch options...' from the flyout menu or the button at the bottom of the palette, in the options panel choose "process color" from the 'color type menu'

that will convert all the swatches to process. spot colors in the swatches palette are represented with a little black dot on the bottom right corner.

Please read this piece of the documentation to learn about spot colors:

Overview of color in Illustrator