An Italian academic claims to have traced the @ symbol to the Italian Renaissance, in a Venetian mercantile document signed by Francesco Lapi on May 4, sent from Seville to Rome, describing the goods and treasures arriving on a ship from the Americas to Spain 1537.
The document is about commerce with Pizarro, in particular the price of an @ of wine in Peru; @ meant amphora (Italian anfora; Spanish and Portuguese arroba). Currently, the word arroba means the at-symbol and a unit of weight.
In this usage, the symbol represents one amphora, a unit of weight and volume in the “each” sense, i.e. “2 widgets à £5.50 = £42.00” is the accountancy shorthand notation in English commercial vouchers and ledgers to the 1990s, when the e-mail usage superseded the accountancy usage. It also is so used in Modern French and Swedish; in this view, the at-symbol is a stylised form of à that avoids raising the writing hand from the page in drawing the symbol; this compromise between @ and à in French handwriting is in street market signs.