+Which one is the "best"?
+
+=back
+
+=head1 GUARANTEES
+
+To ensure price source prices are comprehensible and reproducible, some
+invariants are guaranteed:
+
+=over 4
+
+=item 1.
+
+Price sources will never on their own change a price. They will offer options,
+and it is up to the user to change a price.
+
+=item 2.
+
+If a price is set from a source, it is read only. A price edited manually is by
+definition not a sourced price.
+
+=item 3.
+
+A price should be able to repeat the calculations done to arrive at the price
+when it was first used. If these calculations are no longer applicable (special
+offer expired) this should be signalled. If the calculations result in a
+different price, this should be signalled. If the calculations fail (needed
+information is no longer present) this must be signalled.