Learn Polish: Wysyłka jest droga, bo waga jest duża. - Shipping is expensive because the weight is high.

Yes! While in English we say a weight is 'high' or an object is 'heavy', in Polish the noun waga (weight) is treated as a size or amount. Therefore, it is described as 'big' (duża) or 'small' (mała). Using wysoka (high) or ciężka (heavy) to describe the word waga sounds unnatural.

I remember droga meaning 'road'. Is it doing double duty here?

Exactly. Droga is a homonym. It is the noun for 'road', but in this sentence it is the feminine form of the adjective drogi (expensive), which matches the feminine noun wysyłka (shipping). Context tells you that 'Shipping is expensive' makes much more sense than 'Shipping is a road'.

How would I use the focus particle już in this sentence to say 'Shipping is already expensive'?

You would typically place już right before the word it emphasizes, making it: Wysyłka jest już droga, bo waga jest duża. This highlights that the cost is already high even without adding more weight or extra fees.

Could I swap the clauses and start the sentence with bo waga jest duża?

No, you generally shouldn't start a sentence with bo. If you want to flip the order to say 'Because the weight is high, shipping is expensive', you need to use the slightly more formal word ponieważ instead: Ponieważ waga jest duża, wysyłka jest droga.