I think the shipping-rate issue you're seeing may be a side-effect of how we have to obtain USPS rates for multi-parcel shipments. Of course, shipping a given weight in separate parcels costs more than shipping them together in a single parcel. However, the USPS.gov rate API provides no way to request a total rate for multiple parcels of differing weights at once, and making separate rate queries for each parcel would take too long sequentially and is logistically impractical in parallel. Therefore, we've had to resort to querying for a single, average parcel weight (total order weight divided by number of parcels), then we just re-multiply that rate by the total number of parcels in the order.
If you actually ship each order in a single parcel holding all items ordered together, but you still want some flexibility in the packaging weights/costs depending on the size of items ordered, we would suggest using only our Tubes packaging types (regardless of whether you'd literally ship in tubes or not), since our Tubes can also consolidate all Tube-packaged items into any unoccupied capacity in the largest Tubes defined for products in the same order. If you also set all tangible items with a Capacity of 9999 (an absurdly high quantity that will never be reached), the cost and weight of the largest "Tube" in the order would be factored just once into shipping calculations for the order, and we would obtain a rate for the full order to be shipped as a single parcel.
The "Cart Currency" condition for Shipping Rules only determines when a buyer's cart order would qualify for that Rule -- i.e., if the pricing currency of items in their cart matches the Cart Currency condition for any Shipping Rule(s), the matching Rule(s) would calculate a shipping charge for their order. Selecting "All Currencies" for a Rule simply means that condition is not considered, so the Rule would apply to all orders meeting that Rule's other conditions, regardless of the pricing currency of items in the buyer's cart. All of your products are priced in USD, so your cart's pricing currency will always be USD, and thus your shipping charges will always be applied in USD as well. All buyers worldwide would pay you in USD, and either the payment processor or the buyer's card issuer would automatically convert that payment amount to the buyer's native currency.
Finally, you don't really need any of the country-specific Shipping Rules you've set up, since you already have Rules set for All Countries that you wouldn't need to override on a country-specific basis. Parcel Post, Medial Mail, and UPS Ground are only available within the US, so if you add Rules for those with the All Countries condition, buyers outside the US simply wouldn't see rates offered for those two services.
