This help page explains our shipping calculation settings in detail:
http://www.e-junkie.com/ej/help.shipping.htm
If you're actually shipping by US Postal Service and setting these flat rates to approximate your USPS costs, it would be simplest to just use our USPS rate-lookup calculation method, and define an accurate per-unit shipping Weight for each product and an empty Weight for each package type you'll use. To cover insurance on the $600 item, you could specify that as the Cost for a package type you'd only use for that product.
That said, if you still prefer to go the product-specific flat-rate route, you will probably find this Tips page helpful to get your bearings on the general approach you'd use:
http://www.e-junkie.com/ej/tips.shipping.item-specific-flat-rate.htm
Essentially, that approach is "gaming the formula" where (Weight X Rate) = the amount charged. If you set a domestic Rate of 1.00, your product and/or empty package Weights can be set as equal to the flat amounts you want to charge for domestic shipping, then you can set other country-specific and "rest of the world" Rates as multipliers of those domestic amounts.
First-item/each-add'l flat-rate approaches can be accomplished if you think in terms of a flat per-item amount with a surcharge on the first item -- e.g., $5 first-item + $2 each-add'l would be done as $2 per item plus a $3 surcharge on the first item. In this case, you'd set the first-item surcharge as the Cost or Weight of a package type (Cost does not get multiplied by Rate, but Weight does), then you'd set the relevant product(s) with a Weight equal to their domestic per-item charge, using the relevant package type, with a Capacity of 9999 (so the package Cost/Weight would only kick in once). Products which use the same package type can share unoccupied capacity in each others' packages.
We don't really have any way to do a flat rate for the first two items then each add'l item, but you could do a flat rate per every two items (e.g., 1-2 items, 3-4, 5-6, etc.) by assigning that flat rate as a Weight and/or Cost to a package type, then setting the relevant product(s) to use that package type with a Capacity of 2.
With the approaches explained above, the only way to limit your $600 product to US-only orders would be to set all your non-US and All Countries (rest of the world) shipping rules with a Max. Order Total of 599.99 (while leaving your US rule with no Min./Max. threshold requirements); if a buyer adds that item to their cart and selects a non-US location, their order won't quality for any shipping rules, so their cart would show -error- for the Shipping amount and not allow them to proceed to checkout. However, as you say, adding at least $600 worth of other products would also fall into this condition.
