Message.purchase constructor
Creates a purchase Message with Processig Code of 00 00 00.
Implementation
factory Message.purchase({required int amount, DateTime? dateTime}) {
final x = Message('0300');
final now = dateTime ?? DateTime.now().toLocal();
x.processCode = 0x000000;
x.amount = amount;
x.dateTime = now;
x.posConditionCode = 0x14;
x.set(46, [0x33, 0x30, 0x30]); // '300' in ASCII.
x.set(48, '200003123001a11003456001c'.codeUnits);
x.currency = 364;
x.set(57, '1.4.8.2'.codeUnits);
return x;
}