Venezuelan state-run oil firm PDVSA has begun offering to ship its own oil, figuring in the costs in crude supply deals to help customers who have struggled to hire vessels to carry the country’s oil due to U.S. sanctions, according to company documents seen by Reuters.The United States has blacklisted vessel owners, shipping operators and threatened to sanction any tanker facilitating the country’s oil exports as it tightens restrictions on trade with the South American country.Washington has been trying to weaken socialist President Nicolas Maduro by choking the OPEC member’s oil exports, depriving the government of petrodollars — its main source of revenue.Most shipping firms are avoiding Venezuela because of the sanctions…