(farmers' cooperative received a PLR concluding that payments co-op made for purchases of products from members were PURPIMs for purposes of computing co-op's DPAD; co-op not allowed to file claim for refund due to such reclassification; refund tax year distinguishable from payments at issue in PLR; no intent at time payments made to treat them as PURPIMs and they were treated as closed transactions; reclassification not possible because members' tax years now closed, so issuance of amended 1099 PATRs too late).