Hi, webhead,
I can't give you a standard shipping price, but here are some things that I've had to take into consideration when dealing with shipping costs.
When figuring shipping, you might want to take three things into consideration - postage costs, packing materials, and labor involved to get the product sent. What service or services do you use for shipping? Do you offer only one speed of shipping or do you offer some kind of expedited shipping?
Also, is everything you send going to weigh about the same or is the a lot of diversity? If they are all about the same weight, then you could set a single shipping price. If not, then there will be the issue of whether you want to have more than one shipping price, or simply set your shipping price to cover everything that you normally send. If you are going to set standard shipping prices, make sure that you take into account the cost of shipping to the parts of the US that are furtherst/most expensive for you.
If you haven't been charging anything at all, seems like starting low and keeping simple might be best. Then you can get a feeling for how it is going and what you might want to change.
Best wishes,
Sasha