How to avoid other Ingress players to steal items when I am exchanging items with others

This is listed as a "known issue" on Google's Ingress help pages. That states "It may take up to two weeks for XM to begin to spawn around new Portals."

A single Portal shield makes attacks 6% / 8% / 10% less effective. So if an attack without shields does 100% damage, with one VR shield the damage is 90%, and with two VR shields it'd be 81% (0.9*0.9). So four VR shields would reduce damage to 66% (0.9^4).

The effect is "diminished" because if you would simply add up the protection values, you'd end up with 60%.

Thus, since multiplication is commutative, order doesn't matter.