Formal. Go ahead and dress to impress! Colors are encouraged.
El Salvador's currency is the US dollar, so no need to exchange any currency.
Yes! Uber is available all throughout the city and in the tourist areas like surf city(the beach).However, if you want to go further away to explore the country before the wedding ,contact the bride and she can provide you with trusted taxi services.
El Salvador is warm and humid, so come prepared with light clothes. Bare in mind the wedding is in the mountains at night which might be cooler so bring to cover yourself in case it gets cold.