I have never ordered from them before.  Are they generally considered to be reputable?

I ask because this is one of the items they have listed on their clearance page:

additional shipping charge $20.11

I realize it is "available for immediate delivery", but do they really think someone is going to order that????  Hopefully it is just an error.

Shipping to MI for 10 mags was $14+ which struck me as a little high too, but the item named "additional shipping charge" above is actually an item they have listed for sale on their clearance page - Clearance items.

I don't know anything about the mags, but I've ordered from Outdoor Marksman several times and never had a problem.  A 5 pack of these mags is $59.95 and shipping to me in AZ is 12.29.  That shipping cost strikes me as a bit high for 5 aluminum mags, but who's shipping isn't too high?

Edited to add:  Shipping for one is $11.75 and shipping for 10 is $12.80, so obviously they have a minimum shipping charge, so it makes more sense to wait until you've got a decent sized order.