store.install update failed

I didn't have any trouble and it seemed to have tested fine so this is strange. You are right that we would be comparing a array or object. I don't think there is a need for all that casting though as php will do it for you. There is also a simpler way to get the value from the query as well and I messed up the variable names so I fixed that in the patch since we really should be consistent. Thanks for catching the edge case where this update is applied with no transactions. I didn't consider that.

I'm going to see about updating the handbook that I got the code from so this error doesn't happen elsewhere as well.

I'm not sure if this is connected, but I suspect that store.install may also fail to change the sequences (db_next_id) key properly if a database prefix is used. Here's why:
In store.install,v 1.20 2006/05/18 12:42:24 we have

function store_update_2() {
    ...
      $ret[] = update_sql("UPDATE sequences SET name = '{ec_transaction}_txnid' WHERE name = '{ec_store_transaction}_txnid'");
    ...

but in database.mysql.inc,v 1.53 2006/05/24 20:46:56 we have

function db_next_id($name) {
  $name = db_prefix_tables($name);
  ...
  db_query("REPLACE INTO {sequences} VALUES ('%s', %d)", $name, $id);
  ...

so the sequences table should be prefixed, i.e. we should have
$ret[] = update_sql("UPDATE {sequences} SET name = '{ec_transaction}_txnid' WHERE name = '{ec_store_transaction}_txnid'");
Patch appended which I hope should fix.
NB this is a guess from inspection of the code only: I don't have an old install of ecommerce so I have not observed this to fail or tested the patch.

housekeeping, assume fixed