common online print services. resolution question

for just plain old run of the mill printing 4x6, snapfish is ok for me. But I have multiple pictures that I want to enlarge and was wondering when you upload to these popular online services, does the resolution on the printed picture get decreased??

Or should I find a better lab to use for my better pictures? I'm still a novice at this

What is the resolution of the image when you are uploading your file. I send mine up at 300 ppi for small prints. When you say you are going to enlarge some of these prints what size are we talking about. I've never used snapfish so I can't comment on their procedures but I would think that most of these high volume places take what you send them and print them. Is there a problem with the prints you have been getting back.