Substantive Procedures

Recalculate = calculate again e.g. invoice amounts/discounts/carrying amount
Reperform = perform a procedure again e.g. a bank reconciliation/extraction of a trial balance

Purchases are part of the payables “cycle”. Tests of control on purchases include, for example, checking that invoices are matched to GRNs – see page 73. If the amounts on invoices are checked (agreed to price lists, calculations checked, etc) this is substantive. Cut-off tests on purchases are described on page 95 as an aspect of inventory.
Because financial statements are largely prepared from a “balance sheet” perspective, a risk-based audit approach focuses on substantiating the y/e balance – trade payables – rather than purchases. This is covered on page 89 (where you see the overlap with payments also). If purchases are of a capital nature then audit procedures for non-current assets will also be relevant (page 99).