Most eays way to shop don't overheart PBE servers and make them down is to give everybody all content from game free and pop a message with new content added into accounts + MAYBE a short news section about changes.
Contents: Champions, Skins, Icons, Wards, Emoticons, Borders... I see no point to buy them if PBE is just for testing.
Also remove shop and loot from PBE servers also is a good ideea. Make Champions how have enough shards for mastery at champions to be auto upgraded.