Shop timing
Steam says that several times during each match, players can upgrade cards through the Tox Shop and create combos. The store page does not specify exact timing, prices, upgrade trees, or inventory odds.
Use upgrades to support a plan
An upgrade is most useful when it supports the strategy your deck is already pursuing. Compare the available choices against your current cards and opponent rather than treating an unverified upgrade as universally optimal.
Verify details in game
For exact costs and effects, use the current game interface or an official developer explanation. This guide sticks to the information that Steam publishes.