Learn German: Falls es regnet, stellen wir die Fracht sofort unter. - If it rains, we will shelter the cargo immediately.

It is separable! You can see this because the prefix unter is kicked to the end of the sentence: stellen wir ... unter. The prefix unter- is a variable prefix. When the verb describes a literal, physical action—like putting cargo under a roof—it separates.

If unterstellen is separable here, when would it be inseparable?

Variable prefix verbs tend to be inseparable when their meaning is abstract or figurative. For example, if unterstellen means 'to allege' or 'to insinuate' (putting a blame onto someone), it stays together: Ich unterstelle ihm nichts (I am not insinuating anything about him).

Why does the main clause start with the verb stellen instead of the subject wir?

This is due to the verb-second rule. The entire first clause (Falls es regnet) acts as the first block of information, occupying Position 1. To keep the main verb in Position 2, stellen must come immediately after the comma, followed by the subject wir.

Could I use wenn instead of falls here? What is the difference?

Yes, you could say Wenn es regnet. Both translate to 'if' in this context. However, wenn can mean 'if', 'when', or 'whenever'. The word falls strictly means 'in the event that' or 'in case', which strongly emphasizes the condition.