The customs allowance is enforced
Andorra's low duties are the reason many people come, and the French and Spanish authorities know it. Both run customs posts on the way out and search cars routinely. Allowances are per person and modest — broadly a few litres of spirits, a carton of cigarettes and a capped value of goods.
Exceeding them means duty plus a fine, and in larger cases seizure of the goods. Keep receipts and check the current allowance published by the country you are driving into.
Getting in and out
There is no airport and no railway. Everyone arrives by road from Toulouse, Barcelona or Girona, over passes that reach above 2,000m. In winter, snow chains or winter tyres are legally required when signed, and the passes close.
Andorra is outside the Schengen Area but has no border controls of its own; you will pass through French or Spanish control on the way out, so carry your passport and check that a single-entry Schengen visa is not invalidated by leaving.